Self-limiting heater and resistance material

A self limiting electrical heating device with an electrical resistance material the resistivity of which is changed by more than a power of (10) within a predetermined, narrow temperature interval and which is arranged between electrical conductors connectable to a voltage source, the conductors and the resistance material being enclosed in an electrically insulating cover. The electrical resistance material (2) comprises: (1) an electrically relatively non-conducting crystalline, monomeric substance which melts within or near the predetermined, narrow temperature interval and which constitutes the outer phase, (2) particles of one or more electrically conducting materials(s), distributed in the non-conducting material, (3) one or more non-conducting powdered or fibrous fillers, which are insoluable in the non-conducting material and which have a considerably higher melting point than this material, similarly distributed in the non-conducting material, whereby the weight ratio between components (1) and (3) is from 10:90 to 90:10.

FIELD OF INVENTION 
This invention relates to self-regulating electrical heating devices with 
electrical resistance materials the resistivity of which is changed by 
more than a power of 10 within a predetermined narrow temperature 
interval. 
BACKGROUND 
Known electrical heating devices which, after reaching a critical 
temperature, rapidly decrease their output without the help of 
thermostatic regulation are based on two or more conductors and an 
intermediate resistance material, the resistivity of which starts to 
increase steeply at the critical temperature. Such materials are called 
PTC-materials (Positive Temperature Coefficient). 
Known PTC-materials for self-limiting heating devices consist of 
crystalline polymers with conducting particles distributed therein. The 
polymers can be thermoplastic or crosslinked. In U.S. Pat. No. 3,243,753 
the steep increase of the resistivity is explained by the expansion of the 
polymer leading to interruption of the contact between the conducting 
particles. In U.S. Pat. No. 3,673,121 the PTC effect is claimed to be due 
to phase changes of crystalline polymers with narrow molecular weight 
distribution. 
According to J. Meyer, Polymer Engineering and Science, Nov. 1973, 462-468, 
the effect is explained by an alteration of the conductivity of the 
crystallites at the critical temperature. 
Common for the known PTC-materials is that the resistivity alone is changed 
greatly above the critical temperature while the other physical properties 
generally remain unchanged. The temperature range in which the resistivity 
increases by a power of 10 is usually 50.degree.-100.degree. C. However, 
for many applications it is not satisfactory that the reduction of the 
power per degree is so small and that it is not possible to freely choose 
the temperature interval for the steep increase of the resistivity. 
In an article by F. Bueche in J. of Applied Physics, Vol. 44, No. 1, 
January 1973, 532-533, it is described how, by combining several percent 
by volume of conducting particles in a semicrystalline matrix, a highly 
temperature-dependant resistivity is obtained. This resistivity is changed 
considerably in a small temperature interval around the crystal melting 
temperature. As the non-conducting matrix various hydrocarbon waxes are 
used. According to the article, it is also possible to add so-called 
"mechanical stabilizers", consisting of polymers soluble in the wax, 
whereby for obtaining good results, it is stated to be important that the 
wax and the polymer are soluble in each other, which means that only one 
phase may exist. 
SUMMARY 
The present invention relates to a self-limiting electrical heating device 
with an electrical resistance material, the resistivity of which is 
changed by more than a power of 10 within a pre-determined narrow 
temperature interval and which is arranged between electrical conductors 
connectable to a voltage source, the conductor and the resistance material 
being enclosed in an electrically insulating cover. The device is 
characterized in that the electrical resistance material consists of (1) 
and electrically, relatively non-conducting crystalline, monomeric 
substance which melts within or near the predetermined narrow temperature 
interval and which forms the outer phase, (2) particles of one or several 
electrically conducting materials distributed in the non-conducting 
substance, (3) one or several non-conducting fillers in the form of 
powder, flakes or fibres, which are insoluble in the non-conducting 
material and which have a considerably higher melting point than this 
material similarly distributed in the non-conducting material, whereby the 
weight ratio between the components (1) and (3) is from 10:90 to 90:10. 
Preferably, the weight ratio between the components (1) and (3) shall be 
between 10:90 and 50:50. 
The invention also relates to the electrical resistance material as such.

DETAILED DESCRIPTION OF EMBODIMENTS 
The change in resistivity per degree Celsius for the electrical resistance 
material according to the invention is smaller at lower temperatures than 
within the predetermined narrow temperature interval. The resistivity of 
the previously known compositions of meltable monomeric substances and 
conducting particles is not constant within temperature ranges above the 
interval where the resistivity is rapidly increasing, but drop from its 
maximum by up to a power of 10 per 20.degree. C. According to the present 
invention, it has now been found that the slope below the critical 
temperature interval is less steep and the decrease above is only very 
small if the mixtures contain one or several non-conducting fillers which 
are insoluble in the non-conducting material. It is important that this 
decrease above is as small as possible, since a large decrease may cause 
the resistivity to be so low that the device will develop power again. 
It has further been found that the power development in the compositions 
should not exceed 5 watts per cm.sup.3, preferably not exceed 2 watts per 
cm.sup.3 in order to avoid electrical breakdown. To be able to design 
heating devices in practice, suitable for connection into mains voltages 
of 110 V or 220 V, the resistivity values of the compositions should be 
greater than 10.sup.3 ohm cm, preferably greater than 10.sup.4 ohm cm. The 
compositions according to the invention can easily be adjusted to the 
desired high resistivity values, whereas it is difficult to reach high 
resistivity values with previously known compositions. 
It has further proved to be advantageous if the thermal conductivity of the 
compositions is high. The compositions according to the invention have 
higher thermal conductivity than previously known compositions. 
An advantageous embodiment for the composition according to the invention 
may be a case in which the filler is present in such a amount and shape 
that the mixture below the switching point is composed of separate 
particles surrounded by the components (1) and (2). This facilitates the 
design of heating devices in which it is desired to change the shape of 
the device. 
As the electrically relatively non-conducting, crystalline, monomeric 
substance melting within or near the predetermined narrow temperature 
interval, substances are used which have high resistivity both in the 
solid and the molten state. 
Substances with a melting point interval of a maximum of 10.degree. C. are 
preferred; preferably the melting point interval shall not exceed 
5.degree. C. It is advantageous if the molecular weight of the substances 
is less than 1000, preferably less than 500. Especially suitable and 
preferred substances are organic compounds or mixtures of such compounds 
which contain polar groups, e.g. carboxylic or alcohol groups. Suitable 
polar organic compounds, which are excellent to use as relatively 
non-conducting meltable substances according to the present invention, 
are, for example, carboxylic acids, esters or alcohols. It has been found 
that such polar organic compounds improve the reproducibility of the 
temperature-resistivity curves when the mixtures are repeatadly heated and 
cooled, compared with mixtures with non-polar substances. A further 
advantage of polar organic compounds is that they are less sensitive to 
the mixing conditions as such. 
As component 2, particles of one or several electrically conducting 
materials, such particles of metal, e.g. copper, are used. Further there 
are used particles of electrically conducting metal compounds, e.g. 
oxides, sulfides and carbides, and particles of carbon, such as soot or 
graphite, which can be amorphous or crystalline, silicon carbide or other 
electrically conducting particles. The electrically conducting particles 
may be in the form of grains, flakes or needles, or they may have other 
shapes. Several types of conducting particles can also be used as a 
mixture. Particles of carbon have proved to be suitable. A particularly 
suitable electrically conducting carbon material is carbon black with a 
small active surface. The amount of component 2 is determined by the 
desired resistivity range. Generally the component 2 is used in amounts 
between 5 and 50 parts by weight per 100 parts by weight of component 1. 
When metal powder is used, it may be necessary to use larger amounts than 
50 parts by weight per 100 parts by weight of component 1. 
As component 3, non-conducting powdered, flake-shaped or fibrous fillers 
which are insoluble in the non-conducting substance, there are used, for 
example, silica quartz, chalk, finely dispersed silica, such as 
Aerosil.sup.R, short glass fibres, polymeric materials insoluble in 
component 1, or other inert, insoluble fillers. Especially suitable 
fillers are fillers which are good thermal conductors, e.g. magnesium 
oxide. 
The mixtures of the components (1), (2) and (3) can be made in various 
types of mixers, e.g. in a Brabender mixer or a rolling mill. The mixing 
process is suitably performed at a temperature above the melting point for 
component (1). One or several heat treatments of the mixtures, after the 
mixing process to temperatures above the melting point of the meltable 
substance, causes the temperature-resistivity curves after repeated 
measurements to coincide to a greater extent than without heat treatments. 
The electrical conductors connectable to a voltage source in the 
self-limiting electrical heating device according to the invention may be 
of copper, aluminum or other electrical conductor materials and they may 
be tinned, silver-coated or surface treated in other ways to improve the 
contact properties, the corrosion resistance and the heat resistance. The 
conductors can be solid with round, rectangular or other cross-sectional 
shape. They can also exist in the form of strands, foils, nets, tubes, 
fabrics or other non-solid shapes. 
It is specially advantageous in self-limiting electrical heating devices if 
the electrical conductors connectable to a voltage source are arranged in 
parallel, particularly if an even power output per area unit is desired. 
The narrow temperature interval within which the resistivity of the 
electrical resistance material is drasticly changed is a temperature range 
of about 50.degree. C. at the most, preferably of about 20.degree. C. at 
the most. 
If spacers are used in order to maintain the distance between the 
electrical conductors connectable to a voltage source, when the 
electrically non-conducting material is in the molten state, there can be 
used elements of electrically non-conducting materials, such as glass, 
asbestos or other inorganic materials, cotton, cellulose, plastics, rubber 
or other natural or synthetic organic materials. 
The distance elements can be incorporated in the electrical resistance 
material in the form of wire, yarn, net, lattice or foam material. The 
incorporated distance elements have such a shape or/and packing degree 
that they alone, or together with the insulating cover, prevent the 
electrical conductors connectable to a voltage source from changing their 
relative position when the electrically relatively non-conducting 
resistance material is in the molten state. 
According to one embodiment of the self-limiting electrical heating device 
according to the present invention, the insulating cover alone may 
constitute the distance element by the electrical conductors being 
attached to the cover or by the insulating cover being so shaped that it 
prevents relative movement between the electrical conductors. 
The insulating cover can be of plastic, rubber or consist of other 
insulating materials, e.g. polyethylene, crosslinked polyethylene, 
polyvinylchloride, polypropylene, natural rubber, synthetic rubber or 
other natural or synthetic polymers. 
In the accompanying drawing, FIG. 1 shows a cross-section of a heating 
cable according to the present invention, where the distance between the 
electrical conductors (1), between which an electrical resistance material 
(2) is positioned, is maintained permanently by an insulating cover (3) 
which forms the spacer; 
FIG. 2 shows a cross-section of a heating cable according to the invention, 
where the spacer in the form of glass fibre fabric is incorporated in the 
electrical resistance material (4). 
FIG. 3 shows a cross-section of a heating cable according to the invention, 
where the outer conductor (6) is formed by a copper foil and where the 
spacer in the form of glass fibre fabric has been incorporated in the 
electrical resistance material (4); and 
FIG. 4 shows a cross-section of a heating cable according to the invention, 
where a plastic profile (5) forms the spacer. 
FIGS. 5 and 6 show curves which have been measured in the examples 1-14 for 
the relationship resistivity-temperature. 
The invention will be further illustrated by way of the following examples. 
The procedures in examples 1-14 were as follows: 
The components were mixed in a Brabender mixer for 30 minutes at a 
temperature above the melting point of component (1). The 
temperature-resistivity curves were determined on a rectangular sample 
with silver electrodes on two opposite sides, whereby everything was 
enclosed in a stiff insulating plastic cover. The mean value of the last 
two out of three temperature cycles is described with the exception of 
example 11 (example of comparison), where the third cycle is described. 
Printex 300, Corax L and Flammruss 101 are different carbon black 
qualities. 
EXAMPLE 1 
Stearyl alcohol: 100 parts by weight 
Polyamide (11) powder, Rilsan: 200 parts by weight 
Printex 300 from Degussa: 17.5 parts by weight 
EXAMPLE 2 
Mixture 1 after ageing for 10 days 90.degree. C. 
EXAMPLE 3 
Stearic acid: 100 parts by weight 
Aesosil 200 from Degussa: 15 parts by weight 
Printex 300: 15 parts by weight 
EXAMPLE 4 
Stearyl alcohol: 100 parts by weight 
Magnesium oxide: 150 parts by weight 
Printex 300: 17.5 parts by weight 
EXAMPLE 5 
Stearic acid: 100 parts by weight 
Myanit Dolomit filler "0-10": 400 parts by weight 
Flammruss 101 from Degussa: 50 parts by weight 
EXAMPLE 6 
Stearic acid: 100 parts by weight 
Aerosil 200: 11 parts by weight 
Grafit W-95 from Grafitwerk Kropfmuhl: 30 parts by weight 
EXAMPLE 7 
Stearyl alcohol: 100 parts by weight 
Polymamide 11 powder: 600 parts by weight 
Printex 300: 17.5 parts by weight 
EXAMPLE 8 
Stearic acid: 100 parts by weight 
Silica quartz powder: 250 parts by weight 
Corax L from Degussa: 20 parts by weight 
EXAMPLE 9 
Stearyl alcohol: 100 parts by weight 
Polyamide 11 powder: 400 parts by weight 
Printex 300: 17.5 parts by weight 
EXAMPLE 10 (comparison) 
Stearic acid: 100 parts by weight 
Printex 300: 15 parts by weight 
EXAMPLE 11 (comparison) 
Paraffin, melting point 48.degree.-52.degree. C. 100 parts by weight 
Flammruss 101: 20 parts by weight 
EXAMPLE 12 
Stearic acid: 100 parts by weight 
Silica quartz powder: 150 parts by weight 
Polyamide 11 powder: 100 parts by weight 
Printex 300: 17.5 parts by weight 
EXAMPLE 13 
Stearic acid: 100 parts by weight 
Silica quartz powder: 300 parts by weight 
Grafit W-95: 20 parts by weight 
Printex 300: 8 parts by weight 
EXAMPLE 14 
Stearyl alcohol: 100 parts by weight 
PTFE powder F-510 from Allied Chemical: 200 parts by weight 
Printex 300: 17.5 parts by weight 
EXAMPLE 15 
Between 2 copper foils, 100.times.100 mm, there were placed several layers 
of a glass-fibre fabric impregnated with a mixture of 100 parts by weight 
of methyl stearate, 15 parts of weight of Grafit W-95 and 400 parts os 
weight of chalk. The distance between the copper foils was 10 mm. The 
copper foils were connected to an electrical voltage source of 220 V, 
whereby the laminate was heated. The surface temperature rose to about 
35.degree. C. and remained constantly at this value. The current intensity 
varied depending on how the laminate was cooled. 
EXAMPLE 16 
A cable having a length of 3 m and a cross-section according to FIG. 2 and 
where the distance between the conductors was 15 mm, the thickness of the 
conducting layer 1 mm and its composition the same as in example 9, was 
connected to an electrical voltage source of 220 V. The current intensity 
was 0.5 A when switching on the cable. The cable was put into a heating 
chamber with a temperature of 60.degree. C. The current intensity was less 
than 1 mA, showing that the resistance between the conductors in the cable 
had risen to above 200,000 ohms, the resistivity of the resistance 
material had increased by about 500 times its value at room temperature. 
EXAMPLE 17 
The following compounds were mixed in a Brabender mixer: 
Organic compound (see table): 100 parts by weight 
Aerosil 200: 4 parts by weight 
Silica quartz power: 400 parts by weight 
Printex: 17 parts by weight 
The switching temperature, that is the temperature of which the resistivity 
changes by leaps, was determined. 
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Organic compound 
Switching temperature, .degree.C. 
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Caprylic acid 12 
Capric acid 25 
Lauric acid 40 
Myristic acid 50 
Palmitic acid 57 
Cyclohexanol 18 
Tetradecanol 30 
Methyl stearate 
35 
Phenyl stearate 
45 
Ethyl palmitate 
20 
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