Inventor: ITZHAK YANIV

Title: A NOVEL METHOD FOR THE PRODUCTION OF POROUS CERAMIC MATERIALS

FIELD OF THE INVENTION

This invention relates to a novel method for the production porous ceramic materials, powders and formed bodies, that is very inexpensive and permits to obtain materials having excellent properties, especially low densities and high strength. These materials can be used in many applications, including but not limited to those for which known porous materials are used: for instance, as construction elements, as bricks for high temperature ovens, as elements of armaments, as substrates for metal matrix composites, as adsorbents (e.g. ions, inorganic and organic materials), as ceramic filters, as catalysts and catalyst supports, as sound and heat insulators, as fillers in plastics and composite materials, etc.. Moreover, due to the outstanding properties and the low cost of the products obtained by means of the invention, they can be used in cases in which the materials of the prior art are not satisfactory.

BACKGROUND OF THE INVENTION

The production technology of porous ceramic materials is quite tricky and expensive, and produces, in many cases, materials the performance of which is lower than would be desirable, compared to their cost. For instance, any attempt to lower their density and make the ceramic elements lighter results in structurally weak and unsuitable final products. Also, usually the dimensions of the elements produced vary substantially in the course of the production steps, which makes the process quite difficult to operate.

The following examples from the art of forming porous ceramic materials illustrate the problems that are encountered.

To produce porous ceramic materials, naphthalene can be mixed with the ceramic raw materials. Porous products are obtained on firing. However, this technology has two major drawbacks: a. It is very difficult to distribute the constituents of the mixtures evenly. In most cases, the uneven distribution of the materials is the major cause that limits the strength of the final products, b. The naphthalene must be collected and recycled at a cost.

Another technique requires fine powders of aluminum and basic mixtures of raw materials that can react with the metal during the processing program. This technology has the following drawbacks: it releases the dangerous hydrogen gas and it is limited to those products that tolerate Al ^"4-1-1" . Moreover, the even distribution of the hydrogen fine bubbles in the product, which is an essential element in obtaining high quality porous ceramics, is reached in most cases only by the addition of a variety of surface active agents and with great difficulties. A similar technique, for example, requires the use of fine magnesium powders and acid mixtures of raw materials. Once again the release of dangerous hydrogen gas and its fine distribution in the course of the formation of the porous ceramic materials, are quite serious problems.

There is a variety of other techniques, such as the one that is based on frothing air into ceramic raw materials that contain various surface active agents in varying concentrations, but none of them is satisfying, mainly because of the uneven distribution of materials in the final foamed mixture. Mg ⁺⁺ is included in a large variety of important ceramic materials, such as Magnesia (MgO), Spinel (MgO . AI2O3) and Cordierite (2MgO . 2AI2O3 . 5Siθ2). In some cases, small amounts of Mg ^"1- * ^" are deliberately added to improve the properties of the final ceramic products - e.g. to prevent the formation of large crystals in Alumina (AI2O3); to improve the toughness of Zirconia (Z-O2) and Zircon (ZrSiθ4).

The magnesite cements referred to hereinafter include magnesium oxychloride cements, having a composition defined by nMgO-MgC_2-mH2O. The art deals with compositions in which n = 3 and m = 1, or in which m = 5 and n = 13, or the like. The present invention is not limited to a specific composition, and includes any cement the composition of which comprises MgO and MgCl2, and molecular water. The magnesium cements may also be oxysulfate cements, the composition of which can be described by the formula m'MgO MgSθ4 n'H2θ. Various possible values of n' and m' are known in the art, e.g. n' = 5 and m' = 3. The expression "magnesite cements" also includes mixtures of oxychloride and oxysulfate cements.

Obviously, the structure and compositions of the cements change during the curing or hardening process, in manners that are well known to skilled persons and are discussed in the pertinent literature. When magnesite cements are mentioned in this specification and claims, it should be understood that reference is made to cured or uncured or both to cured and uncured cements, as the case may be.

In the specification and claims, the expression "magnesite cement" is intended to include both magnesium oxychloride or Sorel cement and magnesium oxysulfate cement, and mixtures thereof.

It is a purpose of the present invention to provide an inexpensive and simple method to produce porous ceramic materials of excellent properties.

It is a further purpose of the invention to provide a method to produce said porous ceramic products using common and inexpensive raw materials.

It is a further purpose of the invention to provide a method to produce ceramic fine powders using common and inexpensive raw materials and equipment.

Other purposes and advantages of the invention will appear as the description proceeds.

SUMMARY OF THE INVENTION Surprisingly, it was found that foamed magnesite cements, which are formed by mixing known raw materials (MgO, MgCl2 and/or MgSO4, water and certain organic carboxylic acids (e.g. decanoic acid, nonanoic acid, octanoic acid, 2-ethylhexanoic acid, heptanoic acid, hexanoic acid, pentanoic acid, 3-methylbutanoic acid, 2-methylpropionic acid, butanoic acid, propionic acid, acrylic acid, methacrylic acid, cyclohexylcarboxylic acid, benzoic acid, 4-t- butylbezoic acid, 4-n-butylbenzoic acid, 2-thiophenecarboxylic acid, 3-thiophenecarboxylic acid and mixtures thereof, which are capable of foaming the said mixtures) and/or their anhydrides and/or their salts, can be further fired at temperatures above 400°C to produce bodies of porous ceramic magnesia (MgO). This is rather surprising as the magnesite cements are hydrates, which include large amounts of water molecules that evaporate on warming below ~400°C and lead to dramatic deterioration of the physical properties of any body made of the magnesite cements by warming at such temperatures.

An even more surprising phenomenon was noticed when certain fillers were added to the magnesite cements. Porous ceramic materials with dramatically reduced densities and with high strength were obtained (sometimes, under relatively milder firing conditions than the regular ones). The densities could be varied by modulating the experimental conditions to form objects of different properties. Naturally, varying the composition of the fillers could also affect the kind of ceramics that were obtained. Non limiting examples are: the addition of AI2O3 led to the formation of Spinel (MgO . AI2O3 ); the addition of stoichiometric excess of AI2O3 led to foamed AI2O3 which is sintered by spinel; the addition of Siθ2 and AI2O3 in the right proportions led to Cordierite ( MgO . 5SiO2 . 2AI2O3 ).

This technology is not at all limited to reactive fillers, as mentioned above. Mixing foamed magnesite cements and carbides or nitrides like B4C, TiC, SiC, TiN and Si3N4

and firing the porous green bodies in suitable environment ovens, leads to light porous bodies. The foamed magnesite cements turn into foamed MgO at ~1800°C and form the sintering aid among the particles of the carbides and nitrides. The nature of the sintering aid can be modified as desired. For instance, addition of AI2O3 in the right proportions and firing to ~1500°C leads to Spinel, while the addition of AI2O3 and Siθ2 in the right proportion and firing to ~1450°C leads to Cordierite. The use of regular ovens, which do not exclude oxygen, to sinter e.g. SiC, leads to its partial conversion into Siθ2- Therefore, sintering the SiC with the foamed magnesite cements that contains AI2O3 under such conditions leads to Cordierite at ~1400°C, as a sintering aid.

This invention relates to a novel method for the production porous ceramic materials, which comprises the following steps to obtain e.g. porous magnesia (MgO): a. Mixing the constituents of the foamed magnesite cements: MgO, MgCl2 and/or MgSO4, water and one or more organic carboxylic acids capable of foaming the mixture. b. Shaping the viscous foamed mass into any desired forms by usual methods known in the art (optional). c. Curing the mixture for a short duration to form hard green bodies (optional). d. Reshaping the cured hard green bodies (optional). e. Drying the green bodies at temperatures below 400°C (optional). f. Reshaping the green bodies (optional). g. Firing the foamed mass obtained at any stage above at temperatures above 400°C along suitable thermal programs to produce porous ceramic materials. and h. Reshaping the porous ceramic bodies.

The reshaping in steps e and g may include polishing, shaping, grinding, or other suitable operations.

Naturally, suitable additive(s) and filler(s) can be added, optionally, to the said mixture. They may include additives, such as carbon of suitable particle size distribution, polymers, paraffins, and fatty acids, which decompose entirely on firing. Also, they may include fillers such as MgO (dead burned), Siθ2 , AI2O3 , Zrθ2, ZrSiO ₄ , B4C, TiC, SiC, TiN, Si3N ₄ etc., which either interact with the foamed magnesite cements to form other kinds of ceramics (as the final product or as a sintering aid) or may stay practically intact and constitute (the main) part of the final products.

The invention further provides porous foamed materials, and formed bodies or articles made of such materials.

The invention further provides, as intermediates in the production of porous ceramic bodies, foamed viscous masses comprising mixtures of MgO, MgCl2 and/or MgSO4, water and carboxylic acids, hereinbefore described, which are capable of foaming the said masses. The intermediate foamed viscous masses may, optionally, comprise at least one of the fillers and additives hereinbefore described.

The invention further comprises porous ceramic materials, which are the product the firing of the said foamed viscous masses, and formed bodies, such as cast articles, pellets, injected articles, extruded articles and crushed bodies, made of said porous ceramic materials.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The foamed magnesite cements of the invention comprise the constituents of the cements selected from magnesium oxychloride and/or magnesium oxysulphate cements, together with one or more organic carboxylic acids and/or their anhydrides and/or their salts and/or their acyl halides, which are capable of foaming the said mixtures. The expression "carboxylic acids capable of foaming the said mixtures", as used herein, comprises , for

example, carboxylic acids which pass the "foaming test" described hereinafter in the experimental section or an equivalent test.

Preferably, the carboxylic acid(s) has the formula: R-COOH wherein:

R= Alkyl (linear or branched; saturated or unsaturated; cyclic or acyclic);

Aryl (substituted or unsubstituted); containing up to 10 carbon atoms in each of its straight carbon chains; one or more of its carbon or hydrogen atoms may be replaced by oxygen, nitrogen, phosphorus or sulfur atoms.

In a preferred embodiment of the invention, each carboxylic acid is selected from decanoic acid, nonanoic acid, octanoic acid, 2-ethylhexanoic acid, heptanoic acid, hexanoic acid, pentanoic acid, 3-methylbutanoic acid, butanoic acid, 2-methylpropanoic acid, propionic acid, acrylic acid, methacrylic acid, cyclohexylcarboxylic acid, benzoic acid, 4-t-butylbenzoic acid, 4-n-butylbenzoic acid, 2-thiophenecarboxylic acid, 3- thiophenecarboxylic acid and mixtures thereof. The foaming efficiency of the acids changes with the size of the R group. For instance, the foaming efficiency of propionic acid, its foaming rate and its lower sensitivity to excess amounts of water in the used formulations makes it much superior to that of decanoic acid. However, the decanoic acid lead to higher hydrophobic surfaces and react much milder than the propionic acid. The decision of which acid to use as the foaming agent depends on many factors, and particularly on the desired properties of the final products for the specific intended applications.

According to this invention, the carboxylic acid may or may not polymerize, or may partially polymerize, and when it polymerizes it is dimerized and/or oligomerized and/or polymerized, in situ, during the production of the cements, in the presence or the absence of any added polymerization initiators. The expression "added polymerization initiators" is

meant to indicate effective amounts of such initiators which have been deliberately added, and does not refer to any minor amounts of substances, naturally occurring in the cement components, which may promote polymerization to some extent. It should be noted that polymerizable carboxylic acids, like acrylic and/or methacrylic acids, spontaneously undergo dimerization, oligomerization and/or polymerization, to some extent, under process conditions. The addition of suitable polymerization initiators (e.g. potassium persulfate, sodium perborate, etc.) substantially enhances this phenomenon and under certain conditions no monomeric moiety can be found in the final product. It should be noted that polycarboxylates like polyacrylates are unable to foam the magnesite cement mixtures, but their monomers may foam the said mixtures even while polymerizing during the mixing, as will be demonstrated in the "foaming test".

According to a preferred embodiment of the invention anhydrides of the carboxylic acids and/or their salts can be employed, instead of the acids. Examples of illustrative, but non limitative, salts of the carboxylic acids are the Na ⁺ , Ca ^"1 ^, Mg ^ and Al -* ^" salts. Other salts will be recognized by the skilled chemist, and are not detailed herein, for the sake of brevity.

According to another preferred embodiment of the invention, the cement may further comprise also one or more carboxylic acid(s) and/or anhydrides and/or salts and/or acyl halides, which are not capable of inducing foaming. It should be noted that the general definition of the carboxylic acids include examples that do not lead to foaming, like acetic acid, pyruvic acid, lactic acid, 4-hydroxybutyric acid, malic acid, maleic acid, 4- octyloxybenzoic acid, 4-octylbenzoic acid, citric acid and oxalic acid. The ability of any specific carboxylic acid, within the general group of R-COOH mentioned above, to produce foaming, should and can easily be checked by the skilled person in each instance. A simple test (out of many others that can be devised by persons skilled in the art) for carrying out such a check will be described in the experimental section as "foaming test". Clearly, different foaming tests could be devised and it will be easy, for skilled persons, to determine

when the addition of an organic carboxylic acid produces a cement the density of which is significantly lower than that of a comparable cement prepared without the addition of the acid, showing that this latter is "capable of foaming" the cement..

As will be appreciated by the skilled person, the cements may further comprise conventional additives like carbon powders, polymers, paraffins, waxes, greases, long chain fatty acid esters, silicone rubbers, etc. and/or fillers, including metal oxides, metal hydroxides, metal carbonates, metal carbides, and metal nitrides, which are necessary to produce the desired ceramic products. For the purposes of the present invention, the term "metal" is defined to include silicon and boron. Illustrative, but none limiting, examples of such fillers are: MgO (dead burned), Siθ2 , AI2O3 , Zrθ2, ZrSiθ4, B4C, TiC, SiC, TiN, Si3N4 ₅ etc..

The invention is further directed to porous ceramics and to products made therefrom. Such products include, inter alia, formed bodies comprising a cement according to the invention, which has been fired at elevated temperatures. Such formed bodies include, e.g., cast articles, pelletized articles, structural elements, pressed articles, injected articles, extruded articles, and such articles which have been subsequently crushed, before and/or after firing procedure.

The porous ceramic materials of the invention are useful in similar applications as those of the porous ceramic materials, which are obtained from the existing state of the art processes. Examples of such applications are as follows: as construction elements for special applications, as bricks for high temperature ovens, as elements of armaments, as substrates for metal matrix composites, as adsorbents (e.g. ions, inorganic and organic materials), as ceramic filters, as catalysts and catalyst supports, as sound and heat insulators, as fillers in plastics and composite materials for special applications, etc.. The technology in the present patent application may lead to an extension of the utilization of the porous ceramic products, as it offers a cheaper process and products of improved properties.

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All the above and other characteristics and advantages of the invention will better be understood from the following illustrative and non limiting description of preferred embodiments, with reference to the examples given below.

Experimental Data

Raw Materials

In the examples given hereinafter, the following raw materials were used: - Acrylic acid ofAldrich - P-10

- Propionic acid of Aldrich - P-20

- Hexanoic acid of Aldrich - P-40 -Formic acid ofAldrich

-Acetic acid of Aldrich -E.D.T.A tetra-acid ofAldrich

-Gluconic acid ofAldrich

-Malic acid of Aldrich

-Oxalic acid of Aldrich

-Citric acid ofAldrich -Methacrylic acid of Aldrich

-n-Butanoic acid of Aldrich

-2-Methylpropionic acid of Aldrich

-n-Octanoic acid ofAldrich

-n-Heptanoic acid of Aldrich -3-Methylbutanoic acid of Aldrich

-n-Nonanoic acid ofAldrich

-n-Decanoic acid ofAldrich

-n-Pentanoic acid of Aldrich

-2-Ethylhexanoic acid of Aldrich -4-t-Butylbenzoic acid ofAldrich

-Benzoic acid of Aldrich -4-n-Butylbenzoic acid ofAldrich -Cyclohexylcarboxylic acid of Aldrich

- Palmitic acid ofAldrich

- Stearic acid ofAldrich

- n-dodecanoic acid ofAldrich -Polyacrylic acid of Fluka (#81140)

-Ethylene Acrylic Copolymer of Allied Signal (Grade A-C540; Lot #095406AC)

- Sodium dodecylbenzenesulphonate of Aldrich

- Sodium sulfosuccinate of Cyanamid

- MgO grade "Normal F" of Grecian magnesite - "MgF"

- MgO sintered fine (-200 mesh) of Dead Sea Periclase, Israel - "MgP"

- MgSO4 solution having a density of d=1.2 g/cm^ where the ratio H2O/MgSO4 is 3.1

- MgCl2 solution having a density of d= 1.267- 1.27 g/cm^ were the ratio

H2O/MgCl2=2.61

- SiC (-0.5 mm) of Electro- Abrasives, Buffalo, USA

- B4C powder of Hermann & Starck, Berlin, Germany

- Quartz sand (-200 Mesh) which contains 99.5% Siθ2

- Alumina (C-70 and C-80LS of Alcan Chemicals). The typical properties of the alumina are given in the following table:

Table 1

- --

* - Typical linear shrinkage are usually ~16 %

It should be noted that the purity of the raw materials that can be used may vary, depending of the desired product quality. A variety of commercially available materials can be used as suitable and inexpensive substitutes to those mentioned above.

Production Process

The raw materials, in the desired amounts, were introduced in a Retch Mill type KM-1 and subjected to a grinding/mixing operation for a period of 20 minutes. A viscous mass was thus produced, which was introduced in dies of the dimensions 70x70x20 mm and left there to cure for a period of four days at room temperature. After being cured the cast or bodies were dried at 100°C for twenty four hours and then subjected to a firing program, which depended on the kind of final products. Examples of firing profiles are as follows:

1. The dry green body was warmed in a suitable oven to 400°C at a rate of 30°C hr and then at a rate of 80°C/hr until it reached the maximum firing temperature..

2. The materials were allowed to stay for 30 mins. at the maximum temperature ( e.g. 1800°C for Magnesia, 1400°C for Cordierite and 1500°C for Spinel ). and

3. The oven reached ambient temperature during twelve hours.

The compositions of raw materials that were used are given in the following table.

Table 2

Density o

Weight ( g ) Green Bod

Test MgF Brine Water Al ₂ O ₃ (Type) SiO ₂ MgP P-10 P-20 P-40 g/cm.3

1 25.8 38.7 5.0 65.5 (C-70) 3.0 0.98

2 25.8 38.7 5.0 65.5 (C-80LS) 3.0 1.12

3 11.5 17.5 5.0 31.8 (C-80LS) 46.7 2.0 0.90

4 1 1.5 17.5 5.0 31.8 (C70) 46.7 2.0 0.94

5 20.0 30.0 5.0 40.0 3.0 0.90

6 20.0 25.0 10.0 40.0 3.0 0.85

The brine in Exp. 1-5 is MgCl sol. The brine in Exp. 6 is MgSθ4 sol.

The properties of the final product, after firing, are given in the following table

Table 3

Linear Densit Maximu Product Phase Composition by XR

Shrinkag Firing

Test # % g/cm W.A(%) Temp. °C

1 0 0.65 51.5 1500 Spinel Spinel + Traces of A-2O3

2 0 0.70 56.5 1500 Spinel Spinel + Traces of AI2O3

3 1.0 0.70 75.0 1400 Cordierit Cordierite

4 1.0 0.72 73.5 1400 Cordierit Cordierite

5 0.5 0.72 60.5 1800 Magnesia Magnesia

6 2.0 0.72 55.0 1800 Magnesia Magnesia

Porous Carbides

The compositions of raw materials that were used are given in the following table:

Table 4

Density of

Feed Weight ( g ) Green Bod

Test MgF Brine Water AI2O3 (C-80LS SiC B ₄ C P-20 g/cπ_3

7 5.0 7.5 7.5 5.0 75.0 3.0 1.75

8 5.8 8.8 20.0 3.0 60.0 3.0 1.00

The brine in Exps. - 7,8 is MgCl sol.

The properties of the final product, after firing, are given in the following table.

Table 5

Linear Density W.A Maximum Product Phase Composition by XRD Shrinkage Firing

Test # % g/cm^ % Temp. °C

7 3.3 1.23 45.3 1450 SiC SiC + Traces of Cristobalite

8 5.5 0.62 56.2 1450 B ₄ C B ₄ C

"Foaming Test"

Cement mixtures consisting of the following materials: 60g MgO ("MgF"), 90g MgCl2 brine, 50g quartz sand and the 3.0g of the organic carboxylic acid being tested, are mixed in a laboratory mixer (Retch type KM-=1) for 10 mins.. The mixtures obtained are cast into dies of the dimensions 40x40x160 mm and allowed to cure at room temperature and pressure for 10 days. The specimens are dried at 80°C for 15 hrs. and then their densities are measured. The results are given in the next table 6:

Table 6

Test # Carboxylic Acid Density(g/cm Notes

9 None 1.95 Reference

10 Stearic >1.85 Non-Foaming

1 1 Palmitic >1.85 Non-Foaming

12 Dodecanoic >1.85 Non-Foaming

13 Formic 1.92 Non-Foaming

14 Acetic 1.94 Non-Foaming

15 E.D.T.A 1.93 Non-Foaming

16 Gluconic 1.95 Non-Foaming

17 Malic 1.94 Non-Foaming

18 Oxalic 1.92 Non-Foaming

169 Citric 1.93 Non-Foaming

20 Polyacrylic 1.92 Non-Foaming

21 Ethylene Acrylic Copolym 1.94 Non-Foaming

Table 6 (Continued^

22 Acrylic 0.95 Foaming

23 Methacrylic 0.99 Foaming

24 Propionic 0.82 Foaming

25 n-Butanoic 1.15 Foaming

26 n-Hexanoic 1.23 Foaming

27 2-Methylpropionic 1.18 Foaming

28 n-Octanoic 1.27 Foaming

29 n-Heptanoic 1.30 Foaming

30 3 -Methylbutanoic 1.40 Foaming

31 n-Nonanoic 1.39 Foaming

32 n-Decanoic 1.45 Foaming

33 n-Pentanoic 0.93 Foaming

34 2-Ethylhexanoic 1.28 Foaming

35 4-t-Butylbenzoic 1.12 Foaming

36 Benzoic 0.99 Foaming

37 4-n-Butylbenzoic 1.32 Foaming

38 Cyclohexylcarboxylic 0.88 Foaming

Note 1 : A carboxylic acid is considered capable of foaming the magnesite cement if it produces the said material, according to the above procedure, of a density lower than 1.85 g/cm^ and preferably lower than 1.80 g/cmA

Note 2: Sodium dodecylbenzenesulphonate and sodium dioctylsulfosuccinte gave rise to cements of >1.85 g/cm^ when tested under the above conditions.

While embodiments of the invention have been described by way of illustration, it

will be apparent that the invention can be carried out by persons skilled in the art with

many modifications, variations and adaptations, without departing from its spirit or

exceeding the scope of the claims.