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 pre¬ determined narrow temperature interval.

Known electrical heating devices which, after reaching a cri- tical temperature, rapidly decrease their output without the help of thermostatiσ regulation are based on two or more con¬ ductors and an intermediate resistance material, the resisti¬ vity of which starts to increase steeply at the critical tem¬ perature. 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 USP 3.243.753 the steep increase of the resistivity is ex¬ plained by the expansion of the polymer leading to interrup¬ tion of the contact between the conducting particles. In USP 3.673.121 the PTC effect is claimed to be due to phase changes of crystalline polymers with narrow molecular weight distri- bution.

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 alo¬ ne 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 50-100°C. However, for many appli- • cations it is not satisfactory that the reduction of the po¬ wer per degree is so small and that it is not possible to

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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 combi¬ ning several percent by volume of conducting particles in a semicrystalline matrix, a highly temperature-dependant resis¬ tivity 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 po¬ lymers 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.

The present invention relates to a self-limiting electrical heating device with an electrical resistance material, the re- sistivity of which is changed by more than a power of 10 with¬ in 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 en¬ closed in an electrically insulating cover. The device is cha- racterized in that the electrical resistance material con¬ sists of 1) an electrically, relatively non-conducting crys¬ talline, 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 con- r ducting 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.

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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 mate- rial as such.

The change in resistivity per degree Celsius for the electri¬ cal resistance material according to the invention is smaller at lower temperatures than within the predetermined narrow tem- perature interval. The resistivity of the previously known com¬ positions of meltable onomeric substances and conducting par¬ ticles is not constant within temperature ranges above the interval where the resistivity is rapidly increasing, but drops from its maximum by up to a power of 10 per 20°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. / ^' > r

It has further been found that the power development in the compositions should not exceed 5 watts per cm ³ , preferably not

₃ exceed 2 watts per cm 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 resis¬ tivity values of the compositions should be greater than 0 ohm cm, preferably greater than 10 ohm cm. The compositions

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according to the invention can easily be adjusted to the desi¬ red 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 con¬ ductivity of the compositions is high. The compositions accor¬ ding 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 swit¬ ching point is composed of separate particles surrounded by the components 1) and 2). This facilitates the design of hea- ting 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°C are preferred; preferably the melting point interval shall not exceed 5°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 com¬ pounds or mixtures of such compounds which contain polar groups, e.g. carboxyliσ or alcohol groups. Suitable polar organic com- pounds, which are excellent to use as relatively nό ^' n-conduc- ting meltable substances according to the present invention, are, for example, carboxyliσ acids, esters or alcohols. It has been found that such poiar organic compounds improve the rep- roducibility of the temperature-resistivity curves when the mixtures are repeatadly heated and cooled, compared with mix¬ tures with non-polar substances. A further advantage of polar organic compounds is that they are less sensitive to the mixing conditions as such. ^BljKEAT

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As component 2, particles of one or several electrically con- ^' ducting materials, such particles of metal, e.g. copper, are used. Further there are used particles of electrically con¬ ducting 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 elect¬ rically conducting particles. The electrically conducting par¬ ticles may be in the form of grains, flakes or needles, or they may have other shapes. Several types of conducting par- tides 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 ac¬ tive 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 dis¬ persed silica, such as Aerosil ^R , short glass fibres, polyme¬ ric materials insoluble in component 1 ,or other inert, inso- luble 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 rol- ling mill. The mixing process is suitably performed at a tem¬ perature above the melting point for component 1) . One or se¬ veral heat treatments of the mixtures, after the mixing pro¬ cess to temperatures above the melting point of the meltable substance, causes the temperature-resistivity curves after re- peated measurements to coincide to a greater extent than with¬ out heat treatments.

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The electrical conductors connectable to a voltage source in the self-limiting electrical heating device according to the invention may be of copper, aluminium or other electrical con¬ ductor materials and they may be tinned, silver-coated or sur¬ face treated in other ways to improve the contact properties, the corrosion resistance and the heat resistance. The conduc¬ tors can be solid with round, rectangular or other cross-sec¬ tional 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 hea¬ ting 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°C at the most, preferably of about 20°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-conduc- ting materials, such as glass, asbestos or other inorganic ma¬ terials, cotton, cellulose, plastics, rubber or other natural or synthetic organic materials.

The distance elements can be incorporated in the electrical r 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 con¬ nectable to a voltage source from changing their relative po- sition when the electrically relatively non-conducting resis¬ tance material is in the molten state.

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According to one embodiment of the self-limiting electrical heating device according to the present invention, the insu¬ lating 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 po- lyethylene, 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 main¬ tained 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 fab¬ ric 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 ma¬ terial (4) ; and J 'r.

Fig. 4 shows a cross-section of a heating cable according to the invention, where a plastic profile (5) forms the spacer.

Figures 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 follo¬ wing examples. The procedures in examples 1-14 were as ollows: Q RE Z

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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 rectangu¬ lar sample with silver electrodes on two opposite sides, where¬ by everything was enclosed in a stiff insulating plastic co¬ ver. 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 qua¬ lities.

Example 1

Stearyl alcohol 100 parts by weight

Polyamide (11) powder, Rilsan 200 - " -

Printex 300 from Degussa 17,5 - " -

Example 2

Mixture 1 after ageing for 10 days 90°C.

Example 3

Stearic acid 100 parts by weight

Aesosil 200 from Degussa 15 - " -

Printex 300 15 - " -

Example 4

Stearyl alcohol 100 parts by weight Magnesium oxide 150 - " - Printex 300 17,5 - " -

Example 5

Stearic acid 100 parts by weight

Myanit Dolomit filler "0-10" 400 - " -

Flammruss 101 from Degussa 50 - " -

Example 6

Stearic acid 100 parts by weight

Aerosil 200 11 parts by weight

Grafit W-95 from Grafitwerk Kropfmuhl 30 - " -

Example 7 Stearyl alcohol 100 parts by weight Polymamide 11 powder 600 - " - Printex 300 17,5 - " -

Example 8 Stearic acid 100 parts by weight

Silica quartz powder 250 - " - Corax L from Degussa 20 - " -

Example 9 Stearyl alcohol 00 parts by weight

Pol amide 11 powder 400 - " - Printex 300 17,5 - " -

Example 10 (comparison) Stearic acid 100 parts by weight Printex 300 15 - " -

Example 11 (comparison) Paraffin, melting point 48-52°C 100 parts by weight Flammruss 101 20 - " -

Example 12 Stearic acid 100 parts by weight Silica quartz powder 150 - # - Polyamide 11 powder 100 - * - Printex 300 17,5 - '" -

Example 13 Stearic acid 100 parts by weight Silica quartz powder 300 - " - Grafit W-95 20 - " - Printex 300 8 - " - '

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Example 14

Stearyl alcohol 100 parts by weight

PTFE powder F-510 from Allied Chemical 200 - " -

Printex 300 17,5 - " -

Exam le 15

Between 2 copper foils, 100 x 100 mm, there were placed se¬ veral layers of a glass-fibre fabric impregnated with a mix¬ ture 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, where¬ by the laminate was heated. The surface temperature rose to about 35°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 com¬ position the same as in example 9, was connected to an elect¬ rical 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°C. The current intensity was less than 1 mA, showing that the resistance between the con¬ ductors in the cable had risen to above 200,000 ohms, the re¬ sistivity 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 - " -

Silica quartz power 400 - " -

Printex 17 - " -

The switching temperature, that is the temperature of which the resistivity changes by leaps, was determined.

Organic compound Switching temperature, °C

Caprylic acid 12

Capric acid 25

Laurie 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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