Microporous Thermal Insulation Molding

1. Field of the Invention

The present invention relates to a microporous thermal insulation molding comprising compressed thermal insulation material comprising 30-100% by weight of finely divided metal oxide, 0-50% by weight of opacifier, 0-50% by weight of fiber material and 0-15% by weight of inorganic binder.

2. The Prior Art

U.S. Pat. No. 4,985,163 discloses microporous thermal insulation moldings.

The finely divided metal oxides preferably used in such microporous thermal insulation moldings are pyrogenic silicas including electric arc silicas, precipitated silicas or silicon dioxide aerogels and also analogously prepared aluminum oxides. To achieve good thermal insulation properties these finely divided oxides possess very high specific surface areas which preferably lie in the range of 50-700 m.sup.2 /g (measured by BET, as described in ASTM Special Technical Publication No. 51, p. 1941 ff.). As a result of these large surface areas, these oxides have a very strongly pronounced ability to adsorb polar substances. As is known, they therefore act as very effective drying agents and in the natural atmosphere readily absorb water. Accordingly, the same behavior is shown by the microporous thermal insulation moldings produced therefrom. In practice, therefore, moisture absorption by the microporous heat insulation moldings during storage and use cannot be prevented. If in practical use these microporous thermal insulation moldings are now exposed to a high temperature within a short period of time, there is explosive formation of steam which destroys the structure of the thermal insulation molding. This effect occurs, for example, in microporous thermal insulation moldings used as thermal insulation in radiation heating units for ceramic hotplates, if these radiation heating units are brought to red heat by means of the customary resistance heating elements at the intervals of preferably from 1 to 5 seconds which are now customary for the newest generation of such units. However, these short heating times are unavoidable because of the changed consumer attitude.

It is an object of the present invention to raise the gas permeability of microporous thermal insulation moldings, in particular to increase the diffusion of water vapor from the interior to the surface of the molding and thus to avoid local excess pressure, in the interior of the molding, which would destroy the structure of the molding.

The above object is achieved according to the present invention by providing a microporous thermal insulation molding comprising compressed thermal insulation material comprising 30-100% by weight of finely divided metal oxides, 0-50% by weight of opacifier, 0-50% by weight of fiber material and 0-15% by weight of inorganic binder, wherein at least one surface of the molding has channel pores having a cross-sectional area of the pore of 0.01-8 mm.sup.2 and having an intrusion depth of 5-100%, based on the thickness of the molding, with 0.004-10 channel pores being present per 1 cm.sup.2 of the molding surface.

Thermal insulation moldings according to the invention have the advantage of retaining their structure on shock heating even after extreme water absorption, without the thermal insulation properties of the molding being reduced.

The thermal insulation moldings according to the invention customarily have the following property values:

density 250-350 g/l;

flexural strength>0.15 N/mm.sup.2 ;

compressive strength>1.0 N/mm.sup.2 ;

thermal conductivity<0.025 W/mK; and

electrical resistance>2000 M.OMEGA..

Thermal insulation material used according to the invention has the following composition:

30-100% by weight of finely divided metal oxide;

0-50% by weight of opacifier;

0-50% by weight of fiber material; and

0-15% by weight of inorganic binder.

Preferred compositions comprise:

30-89% by weight of finely divided metal oxide;

10-50% by weight of opacifier;

1-50% by weight of fiber material; and

0-5% by weight of inorganic binder.

Particularly preferred compositions include the following:

50-89% by weight of finely divided metal oxide;

20-40% by weight of opacifier;

5-20% by weight of fiber material; and

0.5-2% by weight of inorganic binder.

Finely divided metal oxides which are preferably used are pyrogenic silicas, including electric arc silicas, low-alkali precipitated silicas, silicon dioxide aerogels, analogously prepared aluminum oxides and mixtures thereof. Preference is given to using pyrogenic silicas, aluminum oxide or mixtures thereof. The finely divided metal oxides have BET surface areas of preferably 50-700 m.sup.2 /g, in particular 70-400 m.sup.2 /g.

Examples of opacifiers are ilmenite, titanium dioxide, silicon carbide, iron(II)/iron(III) mixed oxide, chromium dioxide, zirconium oxide, manganese dioxide, iron oxide, silicon dioxide, aluminum oxide and zirconium silicate, and also mixtures thereof. Preference is given to using ilmenite and zirconium silicate. The opacifiers advantageously have an absorption maximum in the infrared range between 1.5 and 10 .mu.m.

Examples of fiber materials are glass wool, rock wool, basalt wool, slag wool and ceramic fibers as are obtained from melts of aluminum oxide, silicon oxide, and mixtures thereof. Preference is given to using fibers obtained from the melts of aluminum oxide and/or silicon oxide. These fibers preferably have the following dimensions:

a diameter of 0.1-12 .mu.m; and

a length of 1-25 mm.

Inorganic binders which can be used are all binders whose use is known in microporous thermal insulation moldings. Examples of such binders are disclosed in U.S. Pat. No. 4,985,163 which is herewith expressly incorporated by reference. Preference is given to using borides of aluminum, of titanium, of zirconium, of calcium, or to using silicides, such as calcium silicide and calcium aluminum silicide, in particular boron carbide. Examples of further components are basic oxides, in particular magnesium oxide, calcium oxide or barium oxide.

The following factors have proven to be important for the gas permeability and thus for the performance of the channel pores: spatial distribution, cross-sectional area and intrusion depth. The number of channel pores per 1 cm.sup.2 of molding surface is from 0.004 to 10 channel pores, preferably from 0.15 to 0.6. The cross-sectional area of the channel pores is 0.01 mm.sup.2 -8 mm.sup.2, preferably 0.10-0.8 mm.sup.2. The intrusion depth depends on the thickness of the molding and is 5-100%, preferably 40-70%, based on the thickness of the molding, taken from the heated side of the molding.

The production of the compressed thermal insulation moldings of the invention preferably comprises the following process steps:

I. Preconsolidating the thermal insulation mixture based on finely divided metal oxide at pressures from 1 to 5 bar, in particular 2 bar or about 2 bar, and compressing the preconsolidated material into the desired shape at final pressures from 8 to 20 bar, with the thickness of the resulting shaped article being preferably from 10 to 35 mm, in particular from 10 to 15 mm.

II. If desired, heating the compressed body at temperatures from 500.degree. C. to 900.degree. C. (hardening process).

III. Formation of the channel pores

1. during step I or

2. after step I or

3. after step II.

The shaping of the channel pores is here carried out using devices known per se for forming channels, such as, for example, drills, punches or milling cutters; however, preference is given to using embossing stamps.

Examples for the shape of the cross section of the channel pores are circles, triangles, rectangles or squares. If desired, this cross section can also vary over the intrusion depth. The channel pores are preferably circular.

The thermal insulation moldings of the invention are used in all areas in which microporous thermal insulation moldings have hitherto been used, such as, for example, in radiation heating units for heating a plate, in particular a glass ceramic plate, in radiation heating of ovens, in particular baking ovens, in heat radiators or in halogen radiators. Heat radiators possess as the heat source a heating wire or a heating strip, while halogen radiators possess a halogen lamp. Recently, heating by halogen and heating by heating wire or heating strip combined in the same heat radiator has become known.

Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying Examples, which disclose the embodiments of the present invention. It should be understood, however, that the Examples are designed for the purpose of illustration only and not as a definition of the limits of the invention.

were compressed to give a molding having a diameter of 198 mm and a thickness of 13 mm and treated for I hour at 800.degree. C. in a furnace.

The molding obtained was maintained at 30.degree. C. at 93% relative humidity for 40 hours. The water absorption was 11% by weight, based on the total weight of the insulation. This moisture-containing molding was tested as described below:

a) Direct heating: a fully installed radiation heating unit comprising a microporous thermal insulation molding and a heating strip was, as would be required in later use, covered with a thermal insulation plate and heated at a heating power of 1800W for 30-60 seconds.

b) Indirect heating: the microporous thermal insulation molding was heated by means of a heating element mounted above its surface, using a heating power of 1800W for 30-60 seconds.

Result: in both cases the moldings burst open because the water vapor pressure in the interior of the parts was too high.

A microporous thermal insulation molding was produced in accordance with the comparative example and provided, once before and once after the thermal treatment at 800.degree. C., with 0.35 channel pores per 1 cm.sup.2 of molding surface by compressing in pins having a diameter of 0.3 mm.sup.2 and a length of 8 mm.

The moldings thus obtained were heated directly and indirectly in the same manner as in the comparative example.

Result: no bursting occurred according to the invention, whereas in the comparative example bursting was observed. The moldings were not damaged according to the invention.

While an embodiment of the present invention has been shown and described, it is to be understood that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention as defined in the appended claims.