GB2357299A

GB2357299A - Insulation component

Info

Publication number: GB2357299A
Application number: GB9929632A
Authority: GB
Inventors: Ali Paybarah
Original assignee: Ceramaspeed Ltd
Current assignee: Ceramaspeed Ltd
Priority date: 1999-12-14
Filing date: 1999-12-14
Publication date: 2001-06-20
Also published as: GB9929632D0; DE20021176U1

Abstract

An insulation component is made by a process in which a layer 2 of compacted particulate thermal insulation material has a surface provided at least in part with a coating 6. In the process the coating 6 is formed on the surface of the layer 2 of insulation material by applying a plurality of droplets in liquid form, which droplets solidify and inter-diffuse with each other on contact with the insulation material. The droplets may be of ilmenite, silica or iron oxide. The coating may further comprise a microporous thermal insulation matrial which may have a higher melting point than the material of the droplets, eg an oxide, carbide, nitride, boride or silicide.

Description

2357299 INSULATION COMPONENT AND PROCESS FOR ITS PRODUCTION This invention

concerns an insulation component comprising a layer of compacted particulate, for example microporous, thermal insulation material having a surface provided at least in part with a coating. The invention also concerns a process for the production of such an insulation component.

More particularly, the invention concerns an insulation component and process for its production, in which the component forms a base for an electric heater, particularly of radiant form, an electric heating element being supported on or adjacent to a surface of the base.

Such a heater may find particular application in cooking appliances.

The term Imicroporous' is used herein to identify porous or cellular materials in which the ultimate size of the cells or voids is less than the mean free path of an air molecule at NTP, i.e. of the order of 100 nm or smaller.

A material which is microporous in this sense will exhibit very low transfer of heat by air conduction (that is, due to collisions between air molecules). Such microporous materials include aerogel, which is a gel in which the liquid phase has been replaced by a gaseous phase in such a way as to avoid the shrinkage which would occur if the gel were dried directly from a liquid. A substantially identical structure can be obtained by controlled precipitation from solution, the temperature and pH being controlled during precipitation to obtain an open lattice precipitate. other equivalent open lattice structures include pyrogenic (fumed) and electrothermal types in which a substantial proportion of the particles have an ultimate particle size less than 100nm. Any of these materials, based, for example on silica, alumina, other metal oxides, or carbon, may be used to prepare a composition which is microporous as defined above.

optionally a binder may be added to provide increased strength, in which case a heat treatment may be necessary to cure the binder.

A known form of high performance microporous thermal insulation material comprises microporous silica particles compacted to consolidate the material into a handleable form, and includes ceramic fibre or glass filament reinforcement and an opacifier such as titanium dioxide (rutile).

Compacted microporous thermal insulation materials suffer from a disadvantage that exposed surfaces thereof are relatively soft and can be damaged by handling.

Furthermore, such materials undergo shrinkage and 3 degradation when exposed to certain liquids, such as water.

Contact with aqueous solutions containing dissolved minerals. particularly sodium chloride or common salt, also causes problems. When a heating element is secured to the microporous insulation material and energised, the melting point of any deposited salt may be exceeded, whereupon mobilised sodium ions may diffuse and migrate into the structure of expanded silica provided in the microporous material. The melting point of the silica is effectively lowered, leading to accelerated sintering thereof and rendering the microporous insulation structure non-effective.

is The above-mentioned problems do not arise with heaters based on microporous thermal insulation material when the heater is covered with an impervious sheet, such as of glass-ceramic material, as in cooking hob applications.

However there is increasing application for such heaters in cooking ovens and grills without the provision of a glass-ceramic or other impervious covering sheet, one reason being the very high cost of the glass-ceramic plate. Such heaters may particularly be required to be located at the top of an oven or grill, where they are at high risk of being exposed to liquids and particulates emitted from food being cooked, and also to salt used in 4 the food preparation. Furthermore, this location for a heater tends to result in the possibility of loose particles of insulation material dropping onto the underlying food.

Operating temperatures, when an inverted heater is covered with a glass or like cloth or plate, may result in increased possibility of local sintering of the insulation material in the vicinity of the heating element and risk of the element loosening and falling from the insulation material.

At least some of these problems can be ameliorated by providing a fabric covering for the heater comprising glass, ceramic or metal filaments. However it would be desirable to be able to dispense with such a covering.

It has also been proposed to provide a protective coating on the surface of the microporous insulation material.

Flame spraying of a material, such as alumina, has been suggested. However it has been found that, because of the high melting point of alumina, alumina particles when flame sprayed onto the insulation tend to solidify before contacting the surface of the insulation and do not adhere to the insulation.

It is an object of the present invention to overcome or minimise this problem.

According to one aspect of the present invention there is provided an insulation component comprising a layer of compacted particulate thermal insulation material having a surface provided at least in part with a coating, wherein the coating comprises a plurality of droplets which have been applied in liquid form and solidified and inter-diffused with each other on contact with the insulation material.

According to a further aspect of the present invention there is provided a process for the production of an insulation component comprising a layer of compacted particulate thermal insulation material having a surface provided at least in part with a coating, the process comprising forming on the surface the coating comprising a plurality of droplets which are applied in liquid form and which solidify and inter-diffuse with each other on contact with the insulation material.

The thermal insulation material preferably comprises a microporous thermal insulation material.

The coating may further comprise particles of a refractory material having a higher melting point than the material of the droplets. The particles of refractory material may comprise one or more of an oxide, carbide, nitride, boride or silicide. Preferably, the refractory material is selected from alumina, zirconia 6 and titania, and mixtures thereof. The coating may comprise a major proportion by weight of the particles of refractory material and a minor proportion by weight of the droplet material. Thus, the coating comprise from 10 to 95 percent, preferably from 55 to 65 percent, by weight of the refractory material and from 5 to 90 percent, preferably from 35 to 45 percent, by weight of the droplet material.

The material of the droplets may comprise a metal oxide material.

The material of the droplets may be selected from ilmenite, silica and iron oxide (Fe304 and/or Fe203), and mixtures thereof.

The coating may have a coefficient of thermal expansion less than about 1 x 10-9/degree Celsius, preferably less than about 1 X 10-6 /degree Celsius.

The coating may be a thermal, flame, or plasma, spray deposited coating.

The coating may have a thickness of from about 10 to about 200 microns, preferably from about 50 to about 100 microns.

7 The insulation component may comprise a base for an electric heater, such as a radiant electric heater, and an electric heating element may be supported on or adjacent to the surface.

The electric heating element may comprise a corrugated metal ribbon supported edgewise on the surface.

The coating may be provided over at least part of the electric heating element.

The coating may be absent from at least part of the surface directly underlying at least part of the electric heating element.

The coating may have a softening temperature higher than a temperature at which the electric heating element is intended in use to operate. The softening temperature of the coating may be in the range from 1000 to 2000 degrees Celsius, ideally at least about 1200 degrees Celsius, and preferably in the range from 1500 to 1900 degrees Celsius.

As a result of the invention, the surface of the thermal insulation material is provided with an adherent protective coating which, in addition to reducing or minimising contact of the thermal insulation material by liquids and salt materials, also assists in maintaining 8 adhesion of a heating element to or in the insulation material.

For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which:

Figure 1 is a plan view of a radiant electric heater incorporating an insulation component according to the invention; Figure 2 is a perspective view of a heating element as provided in the heater of Figure 1; and Figures 3 and 4 are cross-sectional views of alternative embodiments of the heater of Figure 1.

Referring to the drawings, a radiant electric heater, such as for use in a cooking appliance, comprises a metal dish 1 containing a base layer of well known compacted particulate microporous. thermal and electrical insulation material. For some applications, the metal dish 1 may not be required.

By way of example, the insulation material may comprise:

Pyrogenic silica 49 - 97 % by weight Fibre or filament reinforcement 0.5 20 % by weight 9 Opacifier (e.g. titanium dioxide) 2 - 50 % by weight Alumina 0 - 12 % by weight A well known form of heating element 3, comprising a corrugated metal ribbon 4 with legs 5, is mounted edgewise on the surface of the insulation layer 2 and secured by embedding the legs 5 in the insulation layer 2.

An upstanding peripheral wall 7 of insulation material is provided in the heater. The wall 7 may be separate from the microporous insulation layer 2, as shown in Figure 3, and added to the heater later, or may be integral with the microporous insulation layer 2, as shown in Figure 4.

If required, a well known form of temperature limiter 8 may be provided in the heater.

The surface of the microporous insulation layer 2 is provided with a protective coating 6 which has a softening point higher than a temperature at which the heating element 3 is designed to operate and preferably at least 1200 degrees Celsius. The protective coating is applied by a thermal, flame, or plasma, spray coating technique, which is well known per se to the skilled person.

The coating 6 comprises, as major constituent, a refractory material, in practice in the form of at least one metal oxide material having a high melting point, such as alumina (melting point 2024 degrees C.), titania (melting point 1920 degrees C.), or zirconia (melting point 2700 degrees C.), although other materials can be used. Mixtures of such materials may also be considered.

The coating 6 also comprises, as minor constituent, at least one metal oxide flux (droplet) material having a lower melting point than the major constituent. Suitable flux materials include silica (melting point 1705 degrees C.) and iron oxide (Fe304 (melting point 1595 degrees C) and/or Fe,03 (melting point 1550 degrees C)). Such flux materials are conveniently present in ilmenite, which has been found to be suitable for use as a flux material.

The manner in which the coating is applied results in the flux material remaining molten for a longer time than the refractory material such that the flux material is still in the form of molten droplets when it contacts the insulation material. In this way, the droplets of the flux material are able to solidify and inter-diffuse with one another and with the particles of refractory material on contact with the insulation material to form an adherent and coherent coating.

By way of example, the coating 6 may comprise about 60 percent by weight of alumina as the refractory material and about 40 percent by weight of ilmenite as the droplet forming material. The proportion of 40 percent by weight of ilmenite contains about 13 percent titania, 13 percent silica, 9 percent iron oxide and 5 percent of other oxides.

The coating 6 is preferably applied with the heating element 3 in place, since embedding the heating element after applying the coating 6 may be more difficult and/or may result in damage to the coating 6. Furthermore, the coating 6 assists in adhering the heating element to or in the insulation layer 2. When the coating is applied with the heating element in place, coating material may be absent from at least part of the surface directly underlying the heating element, where the element is in contact with the surface.

A suitable thickness for the coating 6 is from about 10 to 200 microns, preferably from 50 to 100 microns, although a coating fillet of greater thickness may be formed by accumulation at the junction between the heating element 3 and the surface of the insulation layer 2.

It has been found that deposition of the coating material on the heating element 3 has no detrimental effect on the performance of the element. However, deposition of coating material on the element can be minimised by 12 - spraying the coating material substantially normal to the surface of the insulation layer 2.

The resulting coating 6 is arranged to be substantially continuous over the exposed surface of the microporous insulation layer 2 and also the integral wall 7 where provided (Figure 4). As a further alternative, the coating may be arranged to be substantially continuous over the exposed surface of the microporous insulation layer 2 and also over a separate wall 7.

Although the coating 6 exhibits a degree of porosity (about 3 percent when applied by thermal spraying and about 1 percent or less when applied by plasma spraying), it has been found that the coating adequately prevents damage to, or degradation of, the microporous insulation layer 2 when the heater in operation is exposed to water, salt solutions, food particulates etc., such as encountered in cooking ovens or grills. Furthermore, particles of microporous insulation material are prevented from falling from the surface of the insulation layer when the heater is installed for operation at the top of an oven or grill.

As a result of the presence of the flux material with the higher melting point metal oxide material such as alumina, the higher melting point material is well bonded to the surface of the insulation layer 2 when the coating material is applied thereto by thermal, flame, or plasma, spray coating.

Claims

1. An insulation component comprising a layer of compacted particulate thermal insulation material having a surface provided at least in part with a coating, wherein the coating comprises a plurality of droplets which have been applied in liquid form and solidified and inter-diffused with each other on contact with the insulation material.

2. An insulation component as claimed in claim 1, wherein the thermal insulation material comprises a microporous thermal insulation material.

3. An insulation component as claimed in claim 1 or 2, wherein the coating further comprises particles of a refractory material having a higher melting point than the material of the droplets.

4. An insulation component as claimed in claim 3, wherein the particles of refractory material comprise one or more of an oxide, carbide, nitride, boride or silicide.

5. An insulation component as claimed in claim 4, wherein the refractory material is selected from alumina, zirconia and titania, and mixtures thereof.

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6. An insulation component as claimed in any one of claims 3 to 5, wherein the coating comprises a major proportion by weight of the particles of refractory material and a minor proportion by weight of the droplet material.

7. An insulation component as claimed in claim 6, wherein the coating comprises from 10 to 95 percent by weight of the refractory material and from 5 to 90 percent by weight of the droplet material.

8. An insulation component as claimed in claim 7, wherein the coating comprises from 55 to 65 percent by weight of the refractory material and from 35 to 45 percent by weight of the droplet material.

9. An insulation component as claimed in any preceding claim, wherein the material of the droplets comprises a metal oxide material.

10. An insulation component as claimed in any preceding claim, wherein the material of the droplets is selected from ilmenite, silica and iron oxide (Fe304 and/or Fe203), and mixtures thereof.

11. An insulation component as claimed in any preceding claim, wherein the coating has a coefficient of thermal expansion less than about 1 X 10-5 /degree 16 - Celsius, preferably less than about 1 x jo-6/degree Celsius.

12. An insulation component as claimed in any preceding claim, wherein the coating is a thermal, flame, or plasma, spray deposited coating.

13. An insulation component as claimed in any preceding claim, wherein the coating has a thickness of from about 10 to about 200 microns, preferably from about to about 100 microns.

14. An insulation component as claimed in any preceding claim, comprising a base for an electric heater.

15. An insulation component as claimed in claim 14, wherein the electric heater is a radiant electric heater.

16. An insulation component as claimed in claim 14 or 15, wherein an electric heating element is supported on or adjacent to the surface.

17. An insulation component as claimed in claim 16, wherein the electric heating element comprises a corrugated metal ribbon supported edgewise on the surface.

17 -

18. An insulation component as claimed in claim 16 or 17, wherein the coating is provided over at least part of the electric heating element.

19. An insulation component as claimed in claim 16, 17 or 18, wherein the coating is absent from at least part of the surface directly underlying at least part of the electric heating element.

20. An insulation component as claimed in any of claims 16 to 19, wherein the coating has a softening temperature higher than a temperature at which the electric heating element is intended in use to operate.

21. An insulation component as claimed in claim 20, wherein the softening temperature of the coating is in the range from 1000 to 2000 degrees Celsius, ideally at least about 1200 degrees Celsius, and preferably in the range from 1500 to 1900 degrees Celsius.

22. An insulation component substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

23. A process for the production of an insulation component comprising a layer of compacted 18 - particulate thermal insulation material having a surface provided at least in part with a coating, the process comprising forming on the surface the coating comprising a plurality of droplets which are applied in liquid form and which solidify and inter diffuse with each other on contact with the insulation material.

24. A process according to claim 23, wherein the thermal insulation material comprises a microporous thermal insulation material.

25. A process according to claim 23 or 24, wherein the coating further comprises particles of a refractory material having a higher melting point than the material of the droplets.

26. A process according to claim 25, wherein the particles of refractory material comprise one or more of an oxide, carbide, nitride, boride or silicide.

27. A process according to claim 26, wherein the refractory material is selected from alumina, zirconia and titania, and mixtures thereof.

28. A process according to any one of claims 25 to 27, wherein the coating comprises a major proportion by 19 weight of the particles of refractory material and a minor proportion by weight of the droplet material.

29. A process according to claim 28, wherein the coating comprises from 10 to 95 percent by weight of the refractory material and from 5 to 90 percent by weight of the droplet material.

30. A process according to claim 29, wherein the coating comprises from 55 to 65 percent by weight of the refractory material and from 35 to 45 percent by weight of the droplet material.

31. A process according to any one of claims 23 to 30, wherein the material of the droplets comprises a metal oxide material.

32. A process according to any one of claims 23 to 31, wherein the material of the droplets is selected from ilinenite, silica and iron oxide (Fe30, and/or Fe203), and mixtures thereof.

33. A process according to any one of claims 23 to 32, wherein the coating has a coefficient of thermal expansion less than about 1 x 10-5/degree Celsius, preferably less than about 1 X 10-6 /degree Celsius.

34. A process according to any one of claims 23 to 33, wherein the coating is deposited by thermal, flame, or plasma, spraying.

35. A process according to any of claims 23 to 34, wherein the coating has a thickness of from about 10 to about 200 microns, preferably from about 50 to about 100 microns.

36. A process according to any one of claims 23 to 35, wherein the insulation component comprises a base for an electric heater.

37. A process according to claim 36, wherein the electric heater is a radiant electric heater.

38. A process according to claim 36 or 37, wherein an electric heating element is supported on or adjacent to the surface.

39. A process according to claim 38, wherein the electric heating element comprises a corrugated metal ribbon supported edgewise on the surface.

40. A process according to claim 38 or 39, wherein the coating is provided over at least part of the electric heating element.

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41. A process according to claim 38, 39 or 40, wherein the coating is absent from at least part of the surface directly underlying at least part of the electric heating element.

42. A process according to any one of claims 38 to 41, wherein the coating has a softening temperature higher than a temperature at which the electric heating element operates.

43. A process according to claim 42, wherein the softening temperature of the coating is in the range from 1000 to 2000 degrees Celsius, ideally at least about 1200 degrees Celsius, and preferably in the range from 1500 to 1900 degrees Celsius.

44. A process for the production of an insulation component substantially as hereinbefore described with reference to the accompanying drawings.

45. An insulation component whenever produced by the process of any one of claims 23 to 44.