Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – Of inorganic materials
Reexamination Certificate
1998-11-17
2001-01-09
Derrington, James (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
Of inorganic materials
C264S044000, C419S002000, C521S082000, C521S091000, C521S092000, C521S094000, C521S919000
Reexamination Certificate
active
06171532
ABSTRACT:
The present invention relates to the stabilization of foamed starting slip materials. In addition, the invention relates to a process for producing inorganic sintered foam parts.
Open-celled inorganic foams are known. They are produced by a variety of processes having hitherto insurmountable drawbacks and therefore such high costs that they are used only in special cases. The process which is claimed by far the most comprises infiltrating open-celled polymer foams with a slip material containing inorganic particles. The infiltrated polymer foam, usually a polyurethane foam, is carefully dried and the organic constituents are removed by slow controlled heating and the negative comprising inorganic powder is sintered. This is the reason for the complicated, expensive production. Both the drying of the pore structure filled with slip material and the pyrolytic removal of the organic constituents are very time consuming. In addition, the thicknesses of the material are restricted to a few centimeters because of the slow drying and pyrolysis. The production of such foams is described, for example, in DE-A 39 34 496 or EP-A 157 974. EP-A 440 322 describes the complicated technology for producing open-celled ceramic foams by means of an arrangement of rollers for the infiltration and for compressing the infiltrated polymer foams.
A wide variety of applications are known for inorganic foams owing to their high temperature resistance and resistance to various media. Thus, DE-A 37 32 654, U.S. Pat. Nos. 5,336,656, 5,256,387, 5,242,882 and 5,217,939 describe ceramic foams as supports for catalysts, e.g. for flue gas treatment. With their random arrangement of the webs, ceramic foams give a very advantageously low pressure drop together with significantly better mass transfer than extruded honeycombs which, owing to the extrusion technology, can have no webs in the flow direction. This applies particularly when the pore volume is more than 50%, preferably more than 70%, of the total volume of the catalyst support and the webs have thicknesses of less than 1 mm. Low pressure drops are particularly important in the application as supports in flue gas purification (DE-A 35 10 170), in motor vehicle exhaust catalysts (DE-A 37 31 888) or in the application as diesel exhaust filters (EP-A 312 501). Ceramic foams are also often used as filters for purifying very hot melts such as metal melts (U.S. Pat. No. 4,697,632) or for filtering hot gases (EP-A 412 931).
All these applications make use of the production of open-celled foams by infiltration of open-celled polymer foams. The inorganic materials claimed have as wide a variety as the applications. For foams having a low thermal expansion, materials claimed are lithium aluminum silicate or cordierite. Such foams have a particularly high resistance to drastic temperature changes, as is necessary for a motor vehicle exhaust catalyst (JP-A 6 1295 283). For melt filters for metals, on the other hand, inert behavior toward the metal melts is important. Use is here made of &agr;-aluminum oxide, silicon carbide, SiO
2
or, in particular, mixtures thereof (EP-A 412 673). Silicon carbide foams are particularly suitable for the filtration of iron melts or melts of ferrous alloys (WO 88/07403). Silicon nitride is also described for ceramic open-celled foams used for filtration (DE-A 38 35 807). EP-A 445 067 describes Y
2
O
3
-stabilized zirconium oxide or mixed ZrO
2
/Al
2
O
3
ceramics as filters for molten metals.
Apart from the infiltration of polymer foams with inorganic slip materials, followed by drying, binder burnout and sintering, other methods have also become known for producing inorganic foams:
WO 95/11752 describes a process in which metals are chemically deposited on an open-celled polymer foam and after drying and pyrolysis there is obtained an open-celled metal foam which can be converted by oxidation into a ceramic foam. Here too, drying and pyrolysis are very complicated. Drying and pyrolysis are avoided in the process claimed in EP-A 261 070, in which ceramic foams are produced starting from a metal foam, preferably an aluminum foam, and then oxidizing this to form a metal oxide. A disadvantage of this process is that a metal foam has to be produced beforehand in some way. A process for producing metal foams (Fraunhofer-Institut f{umlaut over (u)}r Angewandte Materialforschung, Bremen) starts with aluminum powder into which titanium hydride powder is mixed. The powder mixture is heated in a mold to just above the melting point of aluminum, with the titanium hydride being decomposed and the hydrogen formed foaming the molten aluminum. In this case, which cannot be generalized, the melting point of the aluminum matches the temperature range for decomposition of the titanium hydride.
In other known processes too, hydrogen is used as blowing agent for producing inorganic foams: it is thus known that strongly alkaline alkali metal silicates or alkali metal aluminates can be mixed with a powder of a base metal, preferably aluminum, so that the metal dissolves and hydrogen is generated as blowing gas. After the foams have been dried, they have to be treated with ammonium compounds in order to remove alkali metal ions which have an adverse effect. After sintering, such foams can contain less than 0.5% of alkali metal ions (EP-A 344 284, DE-A 38 16 893).
A “dry” process for producing ceramic foams comprises mixing ceramic powders with products of volcanic eruptions which, on heating to 900-1400° C., foam the resulting melt with gas evolution (JP-A 6 0221 371). Foams produced in this way are used in particular as thermally insulating (closed-celled) building material.
JP-A 2 290 211 describes a process for producing ceramic metal melt filters in which resin particles of various sizes, preferably of foamed polystyrene, are joined together and the interstices are infiltrated with a ceramic slip material. The organic constituents are burnt out after drying at from 500 to 600° C. and the foam is then sintered in air at from 1200 to 1800° C.
Open channels in ceramic foams can also be produced by applying short organic fibers such as cotton, polyamide fibers, acrylic fibers or else inorganic fibers such as graphite fibers to a sticky underlay, applying further fibers with an organic binder, infiltrating the fiber lay-up with inorganic slip material, then drying, pyrolyzing and sinterng it (EP-A 341 230). For this is said to produce foams having a pore volume of less than 35%. Applications are as filters for molten metals.
Finally, it is also known that ceramic foams can be produced by admixing aqueous ceramic slip materials with aqueous polymer dispersions, beating the mixture like cream to give a foam until it has a volume of from 1.5 to 10 times the initial volume, allowing the foam to run into a mold, drying it, burning out the organic auxiliaries and then sintering it (EP-A 330 963). In this process, the proportion by weight of inorganic material is from 65 to 95% and the proportion by weight of dispersion (dry mass) is from 5 to 50%, which has to be removed by pyrolysis. Disadvantages in the use of such open-celled inorganic foams are that relatively large bubbles are also beaten in and that a large proportion of the foam cells are closed. On beating, air is incor-porated, the cells formed are stabilized by the polymer dispersion and only some of them are ruptured on drying.
Attempts to highly fill the reactive components of polyurethane foams with inorganic powders and to react these with one another to directly produce a highly filled open-celled polyurethane foam from which, owing to the open-celled nature, the organic constituents could be burnt out are beset by problems. The molecular weight of the components at the commencement of foaming is so low that the foaming mixture is not elastic enough, resulting in the foam bubbles bursting too early and the blowing gas, CO
2
, escaping largely unutilized. The insufficient elasticity also results in rapid formation of cracks in the mass, from which cracks the blowing gas likewise flows unutilized.
BASF - Aktiengesellschaft
Derrington James
Keil & Weinkauf
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