GB2191668A - Electrical heating tape - Google Patents

Abstract

A conductive composition comprising carbon (eg graphite) particles dispersed in an aliphatic acrylic polymer may be formed into an aqueous dispersion with decabromodiphenyl oxide or antimony trioxide as flame inhibitor and cast onto both sides of a polyamide fabric to form a heating tape. A heating element comprising three parallel circular cross-section tubes 12, 12A, 12B enclosed in a silicone rubber insulating cover 2b applied by extrusion is disclosed. <IMAGE>

Description

SPECIFICATION Carbon conductive film for electric heated tapes This invention relates to conductive carbon film (synthetic) for use for example in the manufacture of electrically energised heated tapes.

An object of the invention is to provide an effective and versatile conductive material which is relatively simple and inexpensive to manufacture and is capable of withstanding adverse conditions of use.

According to the invention therefore there is provided a conductive material comprising graphite/carbon particles dispersed within and held together by a body of aliphatic acrylic polymer of water based solvent.

With this material, desirable conductive and protective properties can be obtained in a particularly simple and convenient manner whilst at the same time it is possible to achieve good resistance to abrasion, to corrosive materials (such as acids and alkalis), and to moisture. Moreover the conductive and protective properties (particularly the electrical conductance or resistance) can be maintained generally constant (or at least can be maintained within an acceptable range) throughout a range of operating conditions (e.g. over a temperature range of say -10"C to 1 50 C).

Further the material may have a high modulus of elasticity and good resistance to hydrolysis.

The polymer and the graphite (which comprise the carbon material identified as Monarch 1300 or Raven 8000) may be uniformly mixed for example by a milling process to give a precursor dispersion.

The precursor dispersion may be processed, to give the conductive material in the form of a flexible film tape, supported or integrated into polyamide textile, and this may be used alone in conjuction with any suitable supporting material and may be formed into a circular, or any other, structure in any suitable manner.

The precursor dispersion is formed by extrusion into film by a casting process, this then being split into narrow strips of 14 or 18 mm width.

Alternatively, a wet process may be used involving the homogeneous dispersion by milling of a predetermined percentage of graphite/carbon in acrylic aliphatic resin, and mixing this with a solvent such as water. The proportion of solvent being determined by the required viscosity. The ratio of graphite/carbon to solid weight of resin determines the conductive properties of the resulting material. A thermoplastic resin having an average 30% solids with a homogeneous dispersion of 25% to 55% of graphite/carbon in proportion to the weight of solid resin may be used.

The resulting solid material is obtained as explained above.

The fabric is constructed to give a square law circuit having a closed path for the electrical energy in which the current functions equally and uniformly due to the ohmic resistance within the heated tapes between the insulated electrodes converted into circular tapes.

Moreover, due to the "excitation state", a higher than normal heat energy distribution over frequencies extending into the far infra red at low and ultra low levels can be obtained.

This extension process arises due to the energising source (electric current) passing through molecules of the graphite/carbon (Monarch 1300 or Raven 8000) particles bonded together with the acrylic aliphatic resin.

Also, in so far as the material has no fire hazard or wettability, it is an ideal medium with its particles and characteristics to form an infra red (far) energy emitter for far infra red radiation (wavelength of the order of microns).

Further, the source of energy is electric (AC/DC), thus preventing static which is absorbed within the molecules of graphite/carbon, this being an allotropic non-metal and non-poiar compound.

The material can be resistant to low temperature (-40"C) and may also be solvent resistant, and extremely compatible when combined with graphite/carbon.

Typical mechanical properties may be as follows: 100% modulus(kg/cm2) at 45% graphites and carbon Monarch 1300 or Raven 8000 blended at 25% to 50% Loading =96 300% modulus (kg/cm2) =200 Tensile strength (kg/cm2) =Longitudinal 7.5 Latitudinal 6.5 Elongation % =average 150 Embrittlement ("C) =-40 to -45 Loss of flexibility ( C) =-35 Hardness (Shore A) =88 Film thickness =0.08 mm average The precursor dispersion composed of the graphites, carbon and selected acrylic aliphatic resin may be coated with a doctor blade over silicone paper to give controlled thickness and to remove excess material coating. The coating can then be dried utilising steam techniques and the resulting film formithe conductive material.

By homogeneously dispersing the components and additives of the precursors, the resin acts as a binder for the graphite/carbon particles, yet maintains points of contact and hence electrical conductance. The acrylic aliphatic resin may represent a solid nett weight of 30% in proportion to say 54.5% in weight ratio of the graphite/carbon. The precursor viscosity may be controlled with water solvents. The thickness of film may be on average 100 microns to 200 microns. In the context of electrical resistance the important factors are the volume and weight relationship of the graphite/carbon and the thickness of the film relative to the area and space between the tapping of electrodes which transfer the supply of electrical energy to the heating area.

Example The acrylic aliphatic resin when converted into a film may have the following physical properties: 100% modulus (kg/cm2) =80 Tensile strength (kg/cm2) =615 Elongation =450 The flexibility at 98% R.H. at 70"C The inserting of insulated electrodes with respect to two circular tapes, the third middle tape acting as a transmitter of energy by tapping, at alternative sections, being spaced accordingly.

The voltage applied to adjoining vertical electrodes related to the electrical resistance (OHM) of the tapes.

The mains volage that may be applied for energising the tape may be either AC or DC. The power output per length of conductive tape is constant related to the transferring or tapping electrodes of at least 10 Amp capacity at lengths via the third middle circular tape.

To ensure the correctwatts/linear meter at the designed temperature, the strip resistivity at this temperature should be calculated and used in equation (1).

SRT1=SRT2(1 +[T1-T2]) where SORT, - strip resistivity at design temperature SRT2 - strip resistivity at 200C T1 - design temperature - temperature coefficient of resistance (ohms/ C) based on resistance at 200C.

T2 - 20"C therefore SRT1 =SRT2(1 +0.002[T1 -200C]) ohms/square m.

Some of the advantages of the above described material include as follows: 1. The risk of fire is eliminated because arcing from wires is completely eliminated. 2. Should a break in a circuit occur, only a small portion of the fabric need cease its heating function. By comparison prior art devices frequently fail to function at all if the circuit is broken at any point, such as by fatigue failure of a heating wire.

Thus, there are other possibilities for heating applications being supplied with AC 115V/240V.

The film, when electrically energised, will give uniform heating over the entire surface area controlled electronically via thermistors as sensing devices at say 65% or even higher temperatures.

Description This invention relates to electric heating tapes of specific electrical circuit.

There is a wide range of uses for heating tape, for example in protecting pipes against frost damage and/or facilitating flow through of liquids whose viscosity is temperature dependent and/or protecting fabricated metal structures from deleterious effects of very low temperatures.

It is an object of this invention to provide an alternative construction for heating tapes that is advantageous in terms of electrical characteristics and/or working life expectancy and/or cost of production.

Electrical supply will be connected simply to each end of the two tapes, being copper tinsel of the circular tape of at least 5 Amp capacity.

Another contributory feature is the composition of the carbon-containing suitable composite polymer material, so it does contribute to electrical combined properties and provides adequate temperature tolerance. Accordingly, it is proposed that an irreversibly heat-setting polymer include acrylic/aliphatic components, preferably between 45% and 55%, say about 50% of the latter by weight. Same will be intimately admixed with a suitable carbon (e.g. Cabot Monarch 1300 or Raven 8000) in a suitable quantity, say 25% to 60% by weight. It can further be convenient to incorporate a further nonflammability inhibitor the antimony trioxide at up to 30% by weight.

Heating tapes hereof are quite simple to manufacture, as will become clear from later description of specific implementation, and can afford substantially uniform electrical characteristics per unit area of the tape.

Additionally, the possibility presents itself of the heating tapes being controlled electronically also with a substantial degree of self-limitation, thus not retaining excessive heat. Arising from this is the fact that the electrical resistance of the carbon-containing said composite polymer material converted into circular tapes can be temperature dependent, i.e. it will consume and convert to heat of less electrical energy at higher temperatures, and such action can be correlated to achieving a desired temperature whatever function is required of the tapes.

Specific implementation of presently preferred embodiments of this invention will now be described, by way of example, relative to the accompanying drawing which shows a heating tape 10 and its parts in a selective diagrammatic manner.

Referring to the drawing, there are three circular tubes, 12, 12A, 128, of narrow diameter (say 15 mm) and length or width of say 100 meters. The latter are themselves in mutually intimate contact, electrical conductivity/resistance characteristics of the layers 14A, 14B can be substantially uniform in the width and in the length of the overall tape. Where those layers join through interstices of the circular tube, tape 12, there can further be a reliable average of such characteristics lengthwise of the overall tape and at intervals defining between them substantial area of tape.

The suitable polymer material is a mixture of an acrylic/aliphatic polymer combined with Cabot Monarch 1300 or Raven 8000, also with an additive such as Decabromodiphenyl Oxide, with the aromatic component normally representing about 20% by weight. Particulate carbon will be intimately admixed with the previously mentioned components in an amount of about 50% by weight, say by milling. A suitable water solvent gives a solid content of about 35%, to determine a satisfactory viscosity, and can further serve as a curing agent during heating for that purpose, typically at or after application to the aforesaid film, tape 12. The resulting carbon-containing polymer material becomes highly cross-linked and goes irreversibly solid at curing.

The cured electrically conductive tapes, having braided insulation, 14A, then have inserted into the centre of the two outside circular tapes, parallel lengthwise, insulating tinsel, 1 6A, 1 6B. These insulating tinsels, 1 6A, 16B, inserted into the centre of circular tapes 12 and 12B, rely on conduction/resistance through the thickness of these tapes 12, 1 2A and 12B. Electrical conductors, tapes 12, 12A, 12B, are also externally insulated, with side bridge tapping at intervals corresponding to twice the intended lengths of the electrically conducting areas to be defined, each side via centre tape being shown staggered into a symmetrical pattern for such definition. These are arranged at lengths conveniently relative to area.

The purpose of the central 1 2A is as a conductor via the bridge tapping and is to feed and return the energy to the conductors, 12 and 12B, along its lengths with the energy of a mains electrical supply. Also it is to utilise regions of the conductive material of the tapes 12A and 12B not being connected to the electrical conductor to provide an inner circuit between those extensions, by tapping, 15, of the primary electrical conduction along the lengths of the areas of those regions in order to give the required power dissipation/heat generation for the overall tape so as to get, for instance, about 13/15 watts per metre, as would apply for resistances of 4000 ohms and currents of 0.04 amps at 220/240 volts, or limited to specific voltage to accomplish the method to program for various electrical voltage supplies.Also this allows obtaining maximum or minimum wattage for the application of the pending design without creating an accumulative inherent reduction of ohms resistance over the length.

Afterwards the tapes are wholly encased in insulating material by extrusion, see 26 in the drawing.

Finally, relative to the drawing, reference is made to end tinsel connectors 16, 1 6A for the feed and return conductors, 12A and 12B. Such end connectors may be made at each end of the circuit.

Some advantageous features of heating tapes hereof have been mentioned before, and to turn to further explanation of the advantages of having this new type of carbon, its uniform presence not only aids achievement of uniform, if required temperature-sensitive, electrical characteristics but also uniform thermal characteristics and the capability to utilise energy economically. Moreover, it has a tendency to reduce static.

Claims (13)

1. The conductive composite polymer comprising carbon/graphite particles dispersed and held together by the body of aliphatic acrylic polymer.

2. The composite as claimed in Claim 1, wherein the polymer is water based solvent.

3. The composite as claimed in Claim 1 or 2, wherein the polymer is aliphatic acrylic.

4. The composite as claimed in 1 to 3, wherein the carbon/graphite particles are present in volume of from 25 to 55% in relation to solid polymer by weight.

5. The composite as claimed in any 1 to 4 contains an inhibitor as Decabromodiphenyl Oxide from 20 to 40% of solid polymer by weight.

6. The composite polymer as claimed in 1 to 5 is integrated into the textile of polyamide quality by the method of casting on both sides of the fabric.

7. The composed fabric as claimed in 1 to 6 is non-flammable.

8. The heated tape construction comprises three circular carbon units, 12, 1 2A and 12B tapes, the outside two containing selective insulated conductors (tinsel).

9. The circular carbon conductors, 10, are each insulated externally.

10. The circular carbon conductor, 12, forms an interconnection at an alternative tapping (bridge), 14A and 148, to feed and return relative energy as the heat conversion forming the first circuit via internal carbon conductor, 12, linking alternatively the tape 1 2A or 12B.

11. The heating tape construction as claimed in 8 to 10 requires the formation of the second circuit, 15, by interconnection of the carbon tape 1 2A or 12B into the carbon tape 12 without linking into the conductor (tinsel).

12. The heating tapes as claimed in 8 to 11 are united together, 26, by an external insulating cover such as selective silicone rubber.

13. The heating tape is substantially described with reference to and as illustrated in figures 10, 12, 12A, 12B, 14A, 14B, 15, 16A, 16B, & 6, ofthe accompanying drawing, and also in an abstract.