Refractory granular embedding composition for electric heating coils

A refractory, granular electrically insulative composition useful as an embedding composition for electric heating coils comprises fused and granulated, possibly ground, difficultly fusible, ceramic and possibly oxide-ceramic material having polyalkylsiloxane, e.g. polymethylsiloxane, or polyarylsiloxane admixed in a proportion of from 0.5 to 5.0% added weight. Use of the composition is suitable with tubular heaters which have been produced with it and which, after manufacture and before being brought into use, have been subjected to a heat treatment at a temperature of at least 65.degree. C, for a period of at least 10 minutes. An electrical heating element comprises a housing which is embedded in the composition wherein the ceramic and possibly oxide-ceramic material has been ground.

The present invention relates to a refractory, granular electrically 
insulative composition suitable for use as an embedding composition for 
electric heating coils, advantageously for use with metal-covered 
electrical heater elements, more especially electrical tubular heaters; a 
method of use of such a composition; and an assembly of such a composition 
and an electrical heating element embedded therein. 
Metal-covered electrical heating elements are used for heating liquids in 
baths, for contact heating in cast or poured metals and for heating 
gaseous media. Conventional tubular heaters have an external, metallic 
tubular wall with a diameter which is greater than 6 mm, with a maximum of 
8.5 mm, a heating coil of metallic resistance material being disposed in 
centred relation on the mean axis of the tubular wall. The interstice is 
filled with a ceramic or oxide-ceramic, small-grain insulating 
composition, advantageously fused magnesium oxide. The quality thereof as 
regards transfer of heat and electrical properties is mainly dependent on 
the electric, insulating embedding composition which has been introduced 
between the heating coils and the tubular wall. The fused magnesium oxide 
which is usually employed for this purpose has an excellent thermal 
conductivity and at the same time high electrical resistivity. The optimal 
values for the thermal conductivity and the electrical resistivity are 
obtained with a final consolidation of the magnesium oxide of 3.1 to 3.2 
g/ml. 
Insulating compositions have been previously proposed wherewith the 
electrically fused and thereafter ground magnesium oxide has additives 
admixed therewith in order to improve the electrical properties at high 
operating temperatures. Such compositions are very efficient, but they 
only completely satisfy the purpose for which they are intended if the 
ends of the tubular heaters are sealed in such a way that no moisture is 
able to penetrate into the magnesium oxide. Several constructional forms 
have been previously proposed for the water-tight sealing of tubular 
heaters. For example, constructions have been previously proposed wherein 
elastic sockets or nozzles are introduced into the open tubular heaters 
and are so compressed by means of a tool that moisture is no longer able 
to penetrate into the end of the tubular heater. It is also known to 
introduce a plastics or a liquid composition into an open end of a tube 
and then to provide a seal in the form of a tubular sleeve of silicone 
rubber, ceramics or PTFE (Teflon - Registered Trade Mark), which then is 
likewise subsequently fixed in position in such a way that any emergence 
thereof from the end of the tubular heater is prevented. With these 
insulating compositions and such tube seals, additional working steps are 
necessary in the manufacture of an electrical tubular heater, which 
involve a considerable additional expense for material as well as the time 
which is involved. Despite this additional expense in the manufacture of 
such known heater elements, the possibility of moisture being able to 
penetrate into the open ends of the tubes immediately after an annealing 
operation is not reliably excluded. Since the magnesium oxide has the 
property of being quite hygroscopic, a danger constantly exists of 
moisture being absorbed, and then electrical values, more especially the 
resistance of the electrical insulation, can decrease strongly and a 
discharge current can rise to impermissible orders of magnitude. 
It is an object of the present invention to provide a refractory, granular 
insulating composition suitable for use as an embedding composition for 
electric heating coils, wherein the above-indicated disadvantages are 
eliminated or substantially reduced. 
According to the present invention, therefore, we provide a refractory, 
granular electrically insulative composition suitable for use as an 
embedding composition for electric heating coils, which composition 
comprises fused and granulated, difficultly fusible, ceramic material 
having polyalkylsiloxane or polyarylsiloxane admixed therewith in a 
proportion of from 0.5 to 5.0% by weight, expressed as an added weight, 
based on the total weight of the ceramic material and fused and 
granulated, difficultly fusible, oxide-ceramic material. 
We have established by tests that the addition of polyalkylsiloxanes or 
polyarylsiloxanes to the remainder of the present composition results in a 
considerable improvement thereof as regards the absorption of moisture and 
therefore also have a favourable influence on the electrical properties 
thereof. 
In a preferred embodiment of the present composition, it comprises fused 
and granulated, difficultly fusible, oxide-ceramic material. 
The polyalkylsiloxane preferably comprises polymethylsiloxane. 
The polyalkylsiloxanes or polyarylsiloxanes, have preferably been admixed 
in a grain size from 20 to 200 .mu. with the remainder of the present 
composition. 
Particularly preferred results are obtained, when the present composition 
is in use, when the ceramic material and, where present, the oxide-ceramic 
material, comprise a high-melting metal oxide, preferably magnesium oxide, 
beryllium oxide, titanium dioxide, or silicon dioxide, or a mixture 
thereof. 
In place thereof, it is, however, also preferable that the ceramic material 
comprise a ceramic silicate material, for example of clay. 
According to a further feature of the present invention, we provide a 
method of use of a composition as defined above, in which method tubular 
heaters which have been produced with the composition, after manufacture 
and before being brought into use, have been subjected to a heat treatment 
at a temperature of at least 65.degree. C for a period of at least 10 
minutes. This heat treatment can essentially be omitted if the 
manufactured tubular heater, as is usually the case, has to be subjected 
in any case to an annealing treatment at relatively high temperature for 
other reasons, for example, for eliminating material stresses, but the 
present method has proved to be necessary to maintain the resistance to 
moisture of the tubular heaters in the method. 
According to a still further feature of the present invention, we provide 
an electrical heating element comprising a housing which is embedded in a 
composition as defined above wherein the ceramic material or the 
oxide-ceramic material has been ground.

The following Example illustrates the present composition. 
EXAMPLE 
Several heating elements were produced with which polymethylsiloxane had 
been admixed, in each case in a proportion between 0.5 and 5.0%, expressed 
as an added weight, based on the total weight of a composition comprising 
fused and granulated, difficultly fusible, ceramic and possibly 
oxide-ceramic material. The heating elements consisted of a tubular wall 
made of steel (St 3402), the embedding composition comprising essentially 
magnesium oxide as a basic composition and modified as indicated above and 
an electrical resistance element of a nickel-chrome alloy in the ratio of 
4.1 (by weight). The embedding composition when poured in was compressed 
to a density of approximately 3.2 g/ml by reduction of the cross-sectional 
area of the tubular wall. The specific surface loading for the heating 
element was 2.1 watt/ml. 
After the cross-sectional area of the tubular wall had been reduced, the 
elements were annealed at a temperature of 800.degree. C in a reducing 
atmosphere. The ends of the tubular heaters were not closed. The 
measurement of the electrical resistance with a direct current of 500 volt 
show a value greater than 10.sup.4 M.OMEGA.. Heating elements produced for 
comparison and without any addition of polymethylsiloxane to the embedding 
composition had the same electrical resistances immediately after 
manufacture. 
After storage for 72 hours in a moist chamber at 90% relative humidity and 
at 30.degree. C, an electrical resistance of greater than 10.sup.4 .OMEGA. 
was measured, showing no change, with the tubular heaters having the 
present embedding composition. The comparison heaters without any addition 
to the basic composition had fallen in value to less than 0.3 M.OMEGA. 
after being kept for 8 hours in the moist chamber. After being stored for 
144 hours and after 262 hours in the said chamber, the measured electrical 
resistance of the tubular heaters, with polymethylsiloxane added to the 
embedding composition, was always still greater than 10.sup.4 M.OMEGA..