CA1291512C

CA1291512C - Fibrous heating element, method of production thereof and fabric heating element made thereof

Info

Publication number: CA1291512C
Application number: CA000538358A
Authority: CA
Inventors: Katsunori Ohgushi; Zenwemon Kitazawa; Masao Hijiri
Original assignee: Toray Industries Inc
Current assignee: Toray Industries Inc
Priority date: 1987-05-29
Filing date: 1987-05-29
Publication date: 1991-10-29
Anticipated expiration: 2008-10-29

Abstract

A B S T R A C T

There is disclosed a fibrous heating element which is very pliable and has high mechanical strength in respect of the abrasion resistance and bend yield resistance, the fibrous heating element comprising a core fiber covered with an electrically conductive layer formed by coating with a solution of a pliable synthetic polymer containing electrically conductive particles. There is also disclosed a fabric heating element produced by forming the fibrous heating element into a woven or knit fabric which is pliable as in the case of fabrics in general and is useful as a heating element not only in or for electric heating blankets or the like but also in industrial fields.

Description

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The present invention relates to a fibrous heating element which can be woven or knitted into fabrics in the same manner as fabric-forming yarns and which can be attached to objects by~ for example, sewing. The invention also relates to a method for the production of such a heating element, and a fabric heating element made o~ the fibrous heating element.

Flexible heating wires comprisiny fine metal wire, have been conventionally used as a means of heating or keeping warm various instruments and devices.
Such heating wires are widely utilized in various products such as electric blankets and carpets; in particular, and their utility is increasing.
Nichrome wires are conventionally used for the heating element. However, in products which must be flexible, such as the above-mentioned household products, either a flexible core having a very fine resistance wire spirally wound thereon or a fabric having carbon particles bonded thereon by a resin blnder are used as a heating element.

For example, Japanese patent publication no.
52 14449 discloses a planar heating element which comprises an electrically conductive cloth of a glass-fiber fabric having tinned copper wires woven therein and coated with silicone typed electrically conductive paint.
However, these planar heating elements are poor in pliability, and they all Eail to meet required characteristics with respect to bend yield resistivity and friction or abrasion resistance. In addition, for apparel use or medical use, they are lacking flexibility.
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There have been several attempts to produce fibrous heating elements which comprise a pliable yarn coated with carbonaceous particles. For example, Japanese patent application Kokai (=laying-open) publication no. 51-109321 discloses a composite filament having a sheath component of a low melting point which is subjected to heating to swell the sheath component, with carbonaceous particles attached to and/or contained in the filament. Thereafter the filament is heat-treated to provide a fibrous heating element having a positive temperature coefficient of resistance. The positive temperature coefficient of resistance provides that no particular temperature control means is needed.
However, particularly where the filaments are of large diameter, it is difficult to ensure that the filament contains sufficient carbonaceous particles to provide a resistance value required for a heating element.
According to an example recited in the reference publication, the filament disclosed has an electric resistance value as high as 107 Q/cm, which is not a satisfactory value for a heating element.

Furthermore, Japanese patent publication no.
58-25086 discloses coating a Eiber of a heat shrinkable polymer with an electrically conductive layer and then heat-treating the fiber to obtain an electrically controllable product fiber having a low resistance value per unit length. However, the invention disclosed in this publication is directed to an improvement in the electrostatic property of carpets, and the electric resistance value of the fiber is of the order of 3.3 x 107 ~/cm, so that the products are not useful as heating elements.
Japanese utility model publication no. 38-1470 discloses a pliable heating e]ement which comprises a ~3 reinforcement fiber having a heating layer coated thereon, wherein the haating layer is an electrically conductive rubber or plastic. ~Iowever, the electric resistivity of the pliable heating element is only 50 to 100 Q/m, and it is ineffective as a heating element.

Japanese utility model publication no. 40-15750 discloses an electrically conductive fiber which comprises an insulator fiber with electrically conductive particles coated thereon by adhesive.
However, in this method it is difficult to adhere carbon particles uniformly, so that electrically conductive fiber having a required electric resistance value cannot be supplied consistently.

Furthermore, Japanese patent publication no.
46-23357 discloses a method of producing an electrically conductive fiber, which comprises adhering a paste-like mixture of polyurethane, solvent and electrically conductive particles on a synthetic filament, and forming an electrically conductive coating by removing the solvent. However, the electric conductive fiber is expected to be anti-electrostatic, and is not suitable as a heating element because of its high electrical resistance value of about 109 Q/cm.

An object of one aspect of the present invention is to provide a fibrous heating element which has an electric resistance value higher than that of metals but lower than anti-electrostatic fibers, has remarkable flexibility, and mechanical strength against bending and abrasion and can be processed, for example, for weaving and knitting, and is useable as sewing yarn or thread.

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An object of a further aspect of the invention is to provide a method for the production of such a fibrous heating element.

An object of a still further aspect of the invention is to provide a fabric heating element produced by forming the above fibrous heating element into a form of fabric, which is pliable like fabrics and can be attached to textile products including clothes and other products by, for example, sewing.

According to the present invention, the above objects have been attained by providing a fibrous heating element produced by coating a core fiber with at least one electrically conductive layer of a polyurethane resin containing carbonaceous particles dispersed therein.

Each electrically conductive layer provides a resistance element having a resistance value greater than comparable values of metal resistance elements but lower than those of anti-electrostatic fibers. The heating element comprising a core fiber coated with at least one conductive layer is pliable and has a high mechanical strength against bending and abrasion.
Therefore, a fabric heating element made of such fibrous heating elements is pliable and may be processed as a normal fabri.c.
Embodiments of the invention are described, by way of example only, with reference to the drawinys in which:

Fig. 1 is a plan view of the structure of a fabric heating element comprising a woven fabric according to the present invention;

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Fig. 2 is a plan view of the structure of a mesh-fabric type heating element comprislng a knit fabric according to the invention;
Fig. 3 is a partly broken-away perspective view, showing an embodiment of a fibrous heating element according to the invention;
Fig. 4 is a sectioned view of this embodiment of fibrous heating element according to the invention Fig. 5 is a partial plan view of a fibrous heating element having an electrode part woven with binding yarn; and Fig. 6 illustrates the manner in which a long fibrous heating element is woven in within a short distance between electrodes.

With reference to the accompanying drawings, the present invention will now be described in detail, and initially in relation to the fabric heating element made of the fibrous heating element of the present invention.

Fig. 1 is a partial plan view, showing an embodiment of a fabric heating element according to the invention.

As shown in Fig. 1, the fabric heating element indicated at 1 comprises a woven fabric formed of electrodes 2 in warps, which comprise a fine copper wire plated with tin, and nonconductive yarns 3 of, for example, a polyester fiber which is somewhat diagrammatically shown in Fig. 1, and for the woofs, fibrous heating elements 4 later to be described in detail and nonconductive yarns 5 similar to the above yarns 3, the yarns 5 being incorporated in a proportion necessary for obtaining a desired heating effect. This fabric can be produced by an ordinary loom.

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Furthermore, the electrodes 2 serve to supply power to the fibrous heating elements 4, the other electrodes 2 having been omitted from Fig. 1.

The fabric heating element l can normally be formed on either one surface or both surfaces with an insulating layer (not shown~ by coating thereon a pliable insulating polymer such as for example polyethylene, silicone resin or the like.

Such an insulating layer can be formed by suitable coating means depending on the particular resin or polymer to be used. Alternatively, both services of the fabric heating element can be covered with a film of a thermoplastic resin which is then heat set to form the insulating ]ayer. The thickness of the insulating layer should of course be determined based on the vo]tage of the power source to be used. In this connection, an insulating resin may advantageously be supplied in a molten condition through a nozzle slit of a melt extruder to at least the wire electrode portion of the fabric heating element, and then optionally the formed layer of the insulating resin is covered with a film of a thermoplastic resin, pressiny then beiny performed ~ith cooled rollers, whereby a fabric heating element can be produced in which the contact portions of the wire electrodes and fibrous heatiny elements can always maintai.n close contact with each other. With the fabric heating element which is thereby formed, there is no danger, even if an external force is applied to bend the fabric heating element during the supply of electric current, of the generation of sparking, and the heating element is extremely safe, so that the above manner of manufacture is advantageous.

The fabric heating element illustrated in Fig.
1 is extremely pliable, so that it is useful not only for electric heating blankets and carpets, clothes, medical auxiliary appliances, bedding, sofa material and so forth, but also for a heating source in a broad ranye of industrial products such as products for de-icing, de-frosting, de-dewing, drying and so forth.

Fig. 2 illustrates the basic structure of fabric heating element 10 produced by Russel knitting in which woofs are laid in stitch. This heating element 10 comprises an electrode part 11 and a heat generating part 12, each of which consists of loop yarns and reinforcing yarns.

The electrode part 11 includes reinforcing yarns 13, which comprise a single wire electrode or electrodes of, for example, a copper wire plated with tin and which is electrically connected to the fibrous heating elements 4 by loop yarns 14. Furthermore, the loop yarns 14 should also preferably comprise electrically conductive yarns.

The heat generating part 12 is formed of reinforcing yarns 15 normally nonconductive yarns such as polyester multifilaments and loop yarns 16.
Furthermore, this knit fabric can be formed as a mesh-type fabric by mesh-knitting with a normal warp knitting machine. The fabric heating element according to the present invention can be produced in any other knit form than that illustrated in Fig. 2 by any of the known knitting methods.

The fabric heating element according to the invention can be made in the form of a mesh knit fabric or a mesh woven fabric by any suitable means, and a coating for electrical insulation will then be provided according to the following methods. The coating can be carried out by dipping the fabric in molten resin or in resin solution. Alternatively, a film of a thermoplastic resin may be applied on both surfaces of the fabric, with the fabric then being heated to the melting point of the resin. By, for example, heating to a high temperature as described above and blowlng air, or forming small holes by rollers provided with pins and then heating to the melting point, it is feasible to provide a coating having mesh openings.

As described above with reference to Figs. 1 and 2, the fabric heating element of the invention comprises a fabric which can be produced on an ordinary loom or knitting machine, so that it possesses much higher pliability than conventional planar heating elements produced by forming a conductive layer on a nonconductive base material.

Moreover, where the fabric heating element of the invention is used for electric heating blankets, carpets, clothes or other similar goods, it can be combined with another material by, for example, sewing, as opposed to conventional planar heating elements which cannot be so combined, so that it is highly advantageous from an industrial point of view.

Fig. 3 shows a partly broken-away perspec-tive view of a fibrous heating element of the invention in which a three strand twisted yarn is used as a core fiber.

The illustrated fibrous heating element comprises a core fiber 20 of three-strand twisted polyester yarn, and electrically conductive layers 21, "~,...~

22 and 23 of a polyurethane polymer having carbonaceous particles dispersed therein, formed to cover the core fiber.

The core fiber in the fibrous heating element of the invention is a fiber of thickness normally within a range of 0.1 to 0.5 mm, more preferably, a range of 0.2 to 0.3 mm, and preferably it is a spun yarn, a double-structured yarn, or a textured yarn. Each of the above yarns has a large area for contact with a synthetic resin or polymer forming the electrically conductive layer and can strongly adhere to the resin, so that with such yarns it is feasible to obtain mechanical strength characteristics such as friction or abrasion resistivity, bend resistance and so forth which are high enough to render the fabric durable for subsequent processing.

For the above-mentioned yarns, it is preferable to use a multi-strand twisted yarn, and particularly a two-strand twisted yarn or a three-strand twisted yarn.
Three-strand twisted yarns in particular provide, on the surfaces thereof, little irregularity due to twisting, so that they can provide a fibrous heating element of high guality.

rrhe above-rnentioned double-structured yarn ~ comprises substantially a non-twisted multi-filament core part, and flock-like short fibers or a substantially non-twisted multi-filament sheakh part wound on the surface of the core multifilament.

When the above-mentioned double structured yarn is made of multi-filaments, it is feasible to minimize the elongation of the core fiber to prevent a change in the electric resistance value which is likely when the æ~.5~ ~

core fiber undergoes an elongation, and thus always achieve a constant calorific value. If the multifilament has a number o~ twists exceeding 100 T/m, then core fibers made thereof generally tend to undergo an undesirably great elongation and bring about a change in the calorific value. Thus, the twist number should preferably be below the above-recited value, and more preferably it should be 60 T/m or below. However, if the core fiber does not have a good bundling property, the average thickness of the yarn tends to become very irregular thereby adversely affecting the evenness of the thickness of the heat generating part or layer.
Accordingly, rather than being completely devoid of twist, the multifilament should be twisted to such an extent that a certain degree of the bundling property can be exhibited, for example, a degree of 10 T/m.

The fiber forming the outer layer of the core fiber should preferably be of a shape suitable for adhesion to an electrically conductive layer. For example, the outer layer may be formed by interlacing a fiber surrounding the core fiber by an air jet, by twisting, to form a double structured outer layer, or by forming loops of a textured yarn or a crimp yarn. For the core fiber in the present invention, a plurality of the above-mentioned fibrous heating elements which are twisted together may be used, whereby it is feasible to lower the resistance value per unitary length.
While the fiber for the core fiber may be any natural fiber or synthetic fiber, the following fibers may be advantageously used depending on the intended use of the fibrous heating element.
Thermoplastic synthetic fibers are advantageously useful for they not only are heat i.5~2 resistant, non-hygroscopic, chemically resistant and less liable to deterioration by heat, but the.y also are capable of undergoing breakage by melting when local overheating has taken place and they then function as a thermostatic fuse. Although as stated above no particular limitation applies to the synthetic resin fiber, preferably the fiber should be a nylon type fiber, a polyester type fiber or a polyolefin type fiber having a definite melting point.

Heat resistant fibers having an indefinite melting point in contrast to the above-mentioned fibers are desirable fibers in that they can provide a heating element for use in a high temperature range. Desirable fibers in this respect are, for example, polyfluoroethylene type fibers and wholly aromatic polyamide fibers. Particularly, the latter fibers can provide high tensile-strength fibrous heating elements, and the heating element is suitable for industriaL use.

For the fiber in the core fiber, in addition to a fiber having an ordinary round cross-section, there may be used fibers having a modified cross-sectional shape to obtain an improved adhesion between the fiber and the conductive layer. Particularly where multifilament fiber is used, it is preferable that the fiber has a modified cross-sectional shape as in multilobal fibers such as, for example, a triangular shape, a Y-letter shape, a T-letter shape, a -~ shape, a star shape or a wedge shape, or a U-letter shape, C-letter shape:, a flat shape or a flattened concavoconvex shape. Fibers having such cross-sectional shapes may be used to form a core fiber either as a group of fibers of the same cross-sectional shape, or a mixture or a fiber blend of fibers of different cross-sectional shapes.
For purposes of the present invention, where a fiber of ~Z,9~

a modified cross-sectional shape is used, the cross-sectional shape should preferably be such that, supposing the width of an open space between adjacent projections to be W, the height of projections to be H, the largest radius to be R, and the cross-sectional area to be A, then H/W 2 0.6, H/R 2 0-7 and A/~R2 < 0.5, and fibers having these requirements can be preferably used for the material for the core fiber accordiny to the invention. If the distance of the open space between adjacent projections or branches W is sufficiently small relative to the height of the projections or branches (or the depth of concavities) H, the fibers can have a higher anchoring property and serve to prevent the conductive layer from being stripped from the open space, and H/W should preferably be 0.6 or above or, more preferably, 0.8 or above. Moreover, preferable fibers are those in which the height of the projection or branch (or the depth of the concavities) is sufficiently great and which have peripheral open spaces at many points, with the longest radius in cross-section being R, and H/R being equal to or greater than 0.7.
Furthermore, for a small amount of the fiber to occupy a large volume, it is preferable for the cross-sectional area of the fiber A to be such that A/~R2 is equal to or smaller than 0.5 or, more preferably, 0.~ or less.
Fibers having such a modified cross-sectional shape as above may be filaments, staple fibers or mixtures thereof.
Also, when the core fiber is formed of a fiber of a synthetic polymer which contains a functional group directly bonded to a base polymer, there is improved adhesion of the fibrous heating element to the condllctive layer.

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The functional group directly bonded to a base polymer referred to above means a functional group bonded to the molecular chain of the polymer forming the fiber, and the functional group includes such as the peroxide group, carboxyl group, sulfoxide group, hydroxide group, amino group, amide group, and quaternary amino group.

Means for forming such functional group includes an oxidation treatment, a decomposition treatment and a plasma treatment, of which the plasma treatment is advantageous from the standpoint of mechanical characteristics.
The oxidation treatment oxidizes the fiber surface with an oxidizing agent and imparts a functional group containing oxygen, and both of the usually liquid-phase oxidation and gas-phase oxidation may be used.
The decomposition treatment increases the terminal functional groups by decomposing the polymer surface, and for example the alkaline decomposition of polyester is a representative treatment of this type.
In each of the above treatments the treatment shou:ld preferably not be operated any further than to have the fiber surface treatad.

For the plasma treatment, any of methods normally used in treating fibers can be used.

With a plasma treatment, there is an increase in the functional groups bonded to molecular chains on the surface (within 3000 ~) of a synthetic resin. By making a selective use of ambient gas, it is possible to form and impart, for example, carbonyl group, carboxyl s~

group, hydroxyl group, hydrooxyperoxide group, amino group, amide group and so ~orth.

Furthermore, it is not necessarily required that the core fibers be in a bundled form, and they may otherwise be dispersed in an electrically conductive layer. In the latter case, a large area of contact can be obtained between the monofilament fibers or fiber groups forming the core fiber and the electrically conductive layer, and yet if a stress is generated in the fibrous heating element the stress can be divided into each individual monofilament fiber or individual group of fibers, so that in this case it is possible to obtain an improved mechanical strength of the fibrous heating element.

To provide a fibrous heating element having structural features as described above, the core fiber may be made either of a yarn which is intact as spun and is then drawn, or of a yarn which has been taken up on a bobbin and is then unwound. When the aforementioned suspension can hardly enter into the space among individual fibers or individual fiber groups, preferably the core fiber may be dipped with the fibers in a loose condition. For looseniny the fibers, an air-jet method or a method utilizing static electricity may suitably be used.

Pretreatment by an agent can be used, in which the agent has af:Einity for both the polyurethane resin and the core fiber.

For the polyurethane resin in the present invention, no particular limitation is applicable and any polyurethane resin can be used provided it retains stable electric resistant properties within the .5~2 temperature range in use (20 to 100C), and melts or softens above the upper temperature limit ln use, but a suitable polyurethane resin is, for example, a polyester type, reaction product of diisocyanate and polyester type polyol. The polyester type polyols are obtained normally ~rom dicarboxylic acid and diol.

The above acid component comprises dicarboxylic acids such as adipic acid, sebacic acid and so forth, to which an aromatic dicarboxylic acid such as terephthalic acid, isokphthalic acid and so forth may possibly occasionally be added.

As for the diol component for obtaining the above polyester type diol, it is normally represented by ethylene glycol, propylene glycol, ~,3-butanediol, caprolactonediol and so forth.

The polyester type polyol includes polyethylene glycol, polypropylene glycol, polybutanediol and so forth.

Also for the isocyanate component, use is normally made of hexamethylene diisocyanate, tolylenediisocyanate, xylenediisocyanate, bis-4-isocyanate phenylmethane, isophoronediisocyanate and so forth.

Fine air bubbles formed in the electrically conductive layer of the fibrous heating element of the invention can improve the pliability.

In the fibrous heating element according to the present invention, the polyurethane resin forming the electrically conductive layer may be made of a cross-linked structure, whereby an improvement can be attained in the mechanical strenyth characteristics, the thermal resistivity and the solvent resistivity.

Cross-linking Reaction Means: If a step for permitting a cross-linking reaction to take place and the above-described step for depositing the suspension on the core fiber are simultaneously carried out, then it is likely ~hat the viscosity is raised through gelation as the reaction proceeds, so that normally after the suspension is deposited onto the core fiber, the cross-linking reaction should preferably take place at the same time as the forming of an electrically conductive layer, or after the forming of an electrically conductive layer is completed.

Means which may be used for the cross-linking reaction are, for example, a radical reaction, a reaction by electron beams and a photo reaction.
Each of the above cross-linking reactions will now be described in greater detail. Cross-linking of the polyurethane resin obtainable from the above exemplified components can be by any method such as that in which there is used a cross-linking agent which provides radicals by abstracting hydrogenatom in the methylene group such as benzoylperoxide, that in which the polymer chain or side chain of the polymer is cut and re-oriented by electron beams such as ~ rays and radial rays, that in which polyurethane prepared with the use O:e a diol having double bonds such as 1,2- or ~ polybutadiendiol is subjected to radiation of light beams to undergo cross-linking, and so forth.

By having the electric conductive layer cross-linked as above, it is possible to improve the thermal 5~

resistivity, the solvent resistivity, the mechanical strength and so forth.

As a solvent for the polyurethane resin, the following solvents and mixtures thereof may be used:
N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, tetrahydrofuran, dioxane. To coat a large amount of the electrically conductive resin uniformly, the weight of solvent should be 1 to 10 times, and preferably 2 to 6 times, the weight of the electrically conductive resin.

Carbon black and/or graphite particles are used as the carbonaceous particles in the present invention.

Carbon blacks which are available on the market, for example, acetylene black, channel black, furnace black and so forth can be used as the carbon blacks. A mixture of these carbon blacks can also be used. Normally, the average particle diameter of carbon black(s) is within a range of 1 to 500 m~, preferably 5 to 300 m~, and more preferably 10 to 200 m~, because of its dispersion property to polyurethane resin.
Particles from graphites which are available on the market, for example, black graphite, scaly yraphite, powder graphite, or artificial graphite can be used as the graphite particles. A mixture of these graphite particles can also be used. Normally, the average diameter of graphite particles is within a range of 0.1 to 100 ~m, preferably 0.2 to 50 ~m, and more preferably 0.5 to 20 ~m are used, because of its dispersion property to polyurethane resin.
The amount of the carbonaceous particles used is preferably 30 to 100 parts by weight or, more .,,",.~

.5~ 2, preferably, 40 to 60 parts by weight, to 100 parts by weight of polyurethane resin. With an amount less than 30 parts by weight, the resistance value of the fibrous heating element becomes too high and is not suitable for a heater. With an amount of more than 100 parts by weight, it is difficult to obtain a uniform resistance value for the fibrous heating element, and the mechanical strengths such as bending resistivity and friction resistivity become low, because the proportion of the polyurethane resin becomes small.

When both carbon black and graphite particles are used for the carbonaceous particles, the weight ratio (carbon black/graphite particles) is preferably to 4, and more preferably 1.5 to 2.5. Over or under the above range of weight ratio, the resistance value of the fibrous heating element becomes too high, and the resistance value of the fibrous heating element cannot be uniform.

To the electrically conductive resin in the present invention, various additional agents can be added, if necessary.

While the fibrous heating element according to the present invention comprises one or more carbonaceous particle dispersion layer(s), 2 to ~ layers shculd preferably be formed by coating so that an irregularity in the fiber diameter can be compensated for and an irregularity in the resistance value can be suppressed.
The concentration of the carbonaceous particles dispersed in the synthetic polymer layer can be varied from layer to layer as required.

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The resistance value of the fibrous heating element of the invention can be freely selectively set within a wide range suitably depending on the content of electric conductive particles in the synthetic polymer layer, and the number and the thickness of conductive layers to be applied by coating. A practical range of the resistance value is on the order of l to 100 kQ/m or, more preferably, 3 to 50 kQ/m. When the resistance value is less than 1 kn/m, the heating effect per unit length becomes too high, and when the resistance value is larger than 100 kQ/m, the heating effect per unit length becomes too low, and the fibrous heating element is not suitable as a heater.
To obtain the above resistance value and desired mechanical strength, the thickness of the electrically conductive layer(s) is preferably 20 to 700 m The thickness of the fibrous heating element is determined from the desired thickness of the electrically conductive layer(s) and the form of the heater product.
The preferred fibrous heating elements have a thickness which can be used as components of fabric, and the range of thickness is 0.3 to 1.5 mm.

The above described fibrous heating element according to the invention can be produced by, for example, the following steps.
Preparation Steps Preparation of the core fiber: So that the core fiber can be prepared continuously, there is provided a yarn having no defects such as knots and so forth.

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Preparation of the resin solution having carbonaceous particles suspended therein (hereinafter referred to as suspension): The resin is dissolved in an appropriate solvent in a manner to obtain a solution viscosity of normally 20 to 100 poise (measured by B type viscometer), then carbonaceous particles are suspended in the resulting solution and sufficiently stirred, and the resulting suspension is placed in a vessel which is closed, except for the yarn path opening, in order to prevent the solvent from being evaporated. The above-mentioned solution viscosity can be suitably selectively determined on the basis of the processability to be within a range in which the carbonaceous particles do not settle.
Coating Steps The above-prepared core fiber is dipped in the suspension while the latter is stirred, taken out of the suspension, and then passed throuyh a die of an appropriate orifice diameter to control the deposited amount of the suspension. In order to enhance the mechanical strength of the heat generating layer, it is necessary for the indivi.dual fibers forming the core fiber to be sufficiently wetted with the suspension, and for this purpose the viscosity and the orifice diameter of the die are appropriately adjusted. Industrially, it is preferable to use such a method in which a core fiber wound on a bobbin is continuously withdrawn by a roller mechanism, and is dipped in the suspension.

After it is processed for coating, the core fiber is continuously subjected to a drying or coagulation solvent removal process.
In the case of drying, the process may normally be effected by a ventilation drying method. To improve the production efficiency various means normally employed for promoting the drying, such as, for example, heating the air to be supplied may be used.

In the case of coagulation, because uniform and fine bubbles are formed in the electrically conductive layer(s), flexible fibrous heating element can be obtained.

Repetitional Coating Step:

To suppress any irregularity in the yarn diameter and/or the resistance value and to obtain fibrous heating elements of uniform characteristics, it is preferable to form a plurality of electrically conductive layers. To achieve this, the above-described coating step (including the solvent removal process) may be repeated. In the case of a drying process to remove the solvent, it is necessary sufficiently to effect the drying so that the resin layer formed in a preceding step does not become dissolved in the suspension in a succeeding coating step.

The fabric heating element according to the present invention can be produced using the above-described fibrous heating element by formlng it into a woven or knit fabric according to usually used methods, and in doing the method is normally so performed as to dispose the fibrous heating element in a portion of the woofs and to dispose.
the wire electrodes in a portion of the warps.

Fabric heating elements comprising a fabric made with the above-obtained fibrous heating elements are of a fundamental structure such that normally there is provided a heat generating part (area) comprising fibrous heating elements and nonconductive yarns between two electrodes.
Thus, it is important to provide means for connecting the electrodes to an external power source.

~,9~ l2 Fig. 5 illustrates one embodiment in which a binding yarn is used to fasten the wire electrodes 2 and the fibrous heating elements 4 together.

The binding yarn indicated at 35 comprises a heat shrunk yarn and is used at the location at which the warp comprises the wire electrodes 2. Weaving is so performed that the binding yarn 35 entwines all woofs which cross the electrode wires 2. More specifically, as shown in Fig. 5, the binding yarn 35 is disposed parallel with the fibrous heating element 4 and woofs 36 o~ a nonconductive fiber to cross over the two wire electrodes 2 with the woofs crossing over the binding yarn 35 strongly to press the wire electrodes 2 and woofs 36 against the warps. After weaving, a heat treatment shrinks the binding yarns, and fastens the wire electrodes and woofs. The above-describecl manner of use of the binding yarn 35 can also be applied in the case of a knit fabric.

With woven or knit fabric heating element as provided above according to the present invention, yarns crossing the wire electrodes make it difficult to take out the electrodes for attaching thereto lead wires for connection to a power source. Means for solving this difficulty will be described below.

According to a first solution in providing a fabric having the fibrous heating element and wire electrodes woven therein and having the surfaces thereof coated with an insulation material, there is applied a coating of a release agent or a covering of a protective layer at the prescribed locations between the wire electrodes and the insulating material.

Although no particular limitation is applicable to the release agent in the present invention, normally use j~, ....

~,~??9~ 2 may be made of a silicone resin type ayent or a fluorine resin type agent. Also, for forming the above-mentioned covering, there may be used, for e~ample, a parting paper having a release agent coated on a rear face thereof, or there may be used a thin conductive foil or sheet, which may be double-folded and attached to the electrodes by any suitable means, for example, by soldering or using a conductive bonding agent. In applying such a covering the portion of the electrodes which is to be taken out for 10 connection with the lead wire may be partially exposed from the woven texture to facilitate the appllcation of the covering.

As an alternative means for the connection of the 15 electrodes to the lead wires, there may be used two terminal plates at least one of which has projections on the surface thereof, these plates being so applied to the faces of the electrode that the projections penetrate the electrode, and the two terminal plates being tightly 20 fastened to each other. According to this method, even if the electrodes are covered with a resin film or sheet, the intended electrical connection can be attained without removing the film or sheet.

AlthGugh the fibrous heating elements may normally be arranged to form parallel circuits as shown in Fig. 6, they may be run in a zigzag path between two e].ectrodes along woofs so that they contact the electrodes intermittently optionally to adjust the calorific value to 30 be generated.

For a modified example of the arrangement of the fibrous heating element, the Eibrous heating element may be used in both of the warp and the woof and provide power-35 source terminals at appropriate locations to thereby provide a fabric heating element. Also, for using the ~,9~.5~.2 fibrous heating element for keeping warm the steering wheel of an automobile or a steering handle of a motorcycle, the heating element may be wound about the steering wheel or handle to form a warming or heating face. As an alternative means of providing the fihrous heating element in a fabric, the element may be used as a sewing yarn or thread and sewed in the fabric.

The planar heating element comprising a fabric according to the present invention ma~ be produced so as to comprise a pattern of unitary heating elements in a fabric material or may be in the form of an elongate product comprising a repeat pattern which may be cut and used in the prescribed necessary length. It is also possible to produce a fabric having strands of the electrode incorporated therein and to cut this heating element along the warp direction into segments in proportion to the desired voltages to be used. In this case, the number of patterns to be woven or knit can be reduced, so that the production cost can be advantageously lowered. Also in this case, electrodes to which lead wires are not connected can be made functioning to make the electric current to respective fibrous heating elements uniform and to form bypass circuits in case of a local failure in electricity conduction.

It is ~easible to incorporate a temperature control device which is known per se into the fabric heating element according to the present invention.
Now, with reference to Examples, characteristics of the, fibrous heating element of the present invention will be described.

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Example 1 Preparation of synthetic-polymer suspension:

100 parts by weight of polyester type polyurethane resin (product of Dainichiseika Color & Chemicals MFG. C0., LTD) was uniformly dissolved in 540 parts by weight of a mixed solvent of methylethylketon (hereinafter referred to as MEK) and dimethyformamide (hereinafter referred to DMF) (weight ratio of MEK/DMF:80/20). 50 parts by weight of carbon black (average particle diameter:~0 m~) and 30 parts by weight of graphic particle (average particle diameter:8.~ ~m) were added and dispersed in the polyurethane solution. As measured by B type viscometer, the solution had a viscosity of 45 poise at 30C.

Coating conditions:

While the above-prepared dipping solution was stirred, two-strand twisted polyester spun yarn of 20-count was dipped in and passed through the solution at a rate of 2 m/min at 20C, and the deposited amount of the solution was adjusted through dies of orifice sizes entered in the following Table 1. The dies were of stainless steel and of a type capable of being divided into two in setting the yarn thereon. Thereafter, the yarn was continuously passed through a hot air dryer maintained at 120C to form an electrically conductive layer containing carbonaceous particles dispersed therein around the core fiber. Data on the appearance and various characteristics determined of respective fiber samples obtained through the above 1st stage drying and solidification are shown in Table 1.

Repetitional coating conditions:
Samples No. 3 and 4 in Table 1 were subjected to a 2nd stage treatment using the same dipping solution and in the same manner as above, but for Sample No. 3 a die of an orifice size of 0.8 mm was used, while for sample No. 4 use was made of a die of an orifice size of 0.7 mm.

Further, Sample No. 4 was subjected to a 3rd stage treatment in the same manner as in the 1st and 2nd stage treatments, and in this 3rd stage, a die of an orifice size of 0.8 mm was used.

For the fibrous heating elements on which the 2nd and 3rd stage treatments were performed, the same determinations as above were conducted and the results of the determinations are recited in the following Table 2.

Table 1 Orifioe Resin Diameter Electric Microscopic ~le diameter deposit of fibrous resistance _ observation 20 ~nt heating Surface Sectional (m~) (g/m) element (kSI/m) structure* structure 1 1.0 0.13 0.62 + 0.15 22.5 ~ 1.8 Consider- Rings of able carbona-ceous particles, observa-ble on periph~

2 0.8 0.12 0.55 ~ 0.15 24.9 + 2.0 do do 3 0.7 0.11 0.49 + 0.11 27.8 + 2.4 do do _ 4 0.5 0.085 0.43 ~ 0.13 31.1 + 3.1 do do .
* Concavoconvex irregularities.

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~able 2 Sample Orifi oe Resin Diameter Electric MicroscoDic ~Re- diameter deposit of fibrous resistan oe observation peated ~mm) amount heating coating (lst/2nd element Surfa oe Sectional 10 times) /3rd) (g/m) (kQ/m) structure* structure _ 30.7/1.0 0.13 0.56 + 0.09 18.8 f 1.2 Not con- Rings of (1) . ~ siderable carbona-ceous particles, observa-ble both on outer periphery layer 20 (41)0'5/0'7 0.11 0.49 + 0.09 25.7 ~ 1.9 do do 40,5/0.7/ 0.13 0.55 + 0.03 13.8 + 0.5 Concavo- Rings of (2) 1.0 convex carbona-irregula- ceous rities, particles, little observa-ble both on outer-most layer and inner layers * Concavoconvex irregularities.

The following were noted from the data shown in the above Tables 1 and 2 relating to repeated coatings:

(1) In comparing Sample No. 1 (coated once), Sample No. 3 (coated twice) and Sample No. 4 (coated three times) which had virtually the same amounts of polymer deposited, it was found that the uniformity of the amount of the urethane resin deposited decreased in order of Sample No. 4 (coated 3 times), Sample No. 3 (coated twice) and Sample No. 1 (coated once only). The same tendency is seen regardillg the diameter of the fibrous heating elements and irregularity of the electric resistance per unit length.
(2) In comparing the electric resistance values of fibrous heating elements having the same amount of polymer deposit, it was found that the electric resistance values were lower for each additional coating.
(3) B~ repetitional coating, irregularity on the surface of the elements was reduced and the surface smoothed, so that the friction coefficient of the heating element could be limited enabling the element to be subjected to weaving or knitting processes.

The above Sample No. 2 was subjected to a 2nd stage coating with a solution having 8.3 wt% for the concentration of carbonaceous particles suspended in the solution, and 16.7 wt% for the concentration of the polymer to the solvent, and the characteristics of the treated fibrous heating element were determined to obtain results as shown in Table 3.

Table 3 __ ~le Deposit Electric ~repeated amount o~ resistance ~face structure coatingresin (Smx~hness) times)(g/m) (kQ/m) .
2 0.14 17.5 ~ 0.9 S~rior t4 (1) sample No. 2 (2 t~s) ar~d ~ple Mb. 4 (2 ~es) ..

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From the above Table 3, it is seen that this fibrous heating element (Sample No. 2) has good adhesion properties in the resin treatment after weaving or knitting.

In connection with Sample No. 3 recited in Table 2 and, for comparative samples, Nichrome wire and a commercially obtained cord heater, the bend strength and the friction resistivity were determined to obtain results as entered in the following table 4.

Table 4 .
Resistance Times of Times of value b~ abrasion ~/m) before break before break Nichrome wire (O.~m~ 154 2-3 2 ~0.32mm 18 3 C~m~rci.ally obtained 46-48 200-300 cord heater (2.1rrlT~
Fibrous heating13,000-14,0003,000 5,000122 el~t ~0.56mm From the above Table 4, it is seen that the fibrous heating element according to the present invention is far more durable than the conventional wire heater.

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Example 2 Three different solutions having a carbonaceous particles concentration of 12 wt%, 10 wt% and 5 wt% were prepared in the same procedure as Example 1, wherein the ratio (carbon black/graphite particles) was 2/1.

A core fiber of three-strand twisted polyester spun yarn (30-count) was dipped in and passed through the above suspension containing 12 wt% of carbonaceous particles maintained at 20C at a rate of 2 m/min, then the deposited amount of the dipping solution on the core fiber was adjusted through a die, and the yarn was continuously dried through a drier at a temperature maintained at 120C, to obtain a fiber coated with a layer having carbonaceous particles dispersed therein. The above treatment was repeated using the suspension containing 10 wt% of carbonaceous particles and then the suspension containing 5 wt% of carbonaceous particles, to obtain a fibrous heating element having 3 coating layers having carbonaceous particles dispersed therein.

The fibrous heating element thus obtained was found to be very pliable, to have high bending resistivity and friction resistivity, and to have 12.8 KQ/m as its electric resistance value.

Example 3 A double-structured yarn (0.6mm) consisting of polyester multi-filament (75D-25fil) as the core part and polyester staples (3d, 1.5 inch) as the sheath part wound on the surface of the core multi-filament was used as a core fiber. Except for the core fiber a fibrous heatiny : element was obtained by the same process as Example 2.

The fibrous heating element thus obtained was found to be very pliable, to have high bending resistivity and friction resistivity, and to have 10.8 KQ/m as its electric resistance value.

Example 4 A three-strand twisted polyester textured yarn the cross-sectional shape of which is of 8 leafs type (0.56mm) was used as a core fiber. Except for the core fiber, a fibrous heating element was obtained by the same process as in Example 2.

The fibrous heating element thus obtained was found to be very pliable, to have high bending resistivity and friction resistivity, and to have 14.2 KQ/m as its electric resistance value.

Example 5 A two-strand twisted wholly aromatic-polyamide spun yarn ~20-count, 0.56 mm) was used as a core fiber.
Except for the core fiber, a fibrous heating element was obtained by the same process as in Example 2.

The fibrous heating element thus obtained was found to be very pliable, to have high bending resistivi.ty and friction resistivity, and to have 11.6 KQ/m as its electric resistance vaLue.

Example 6 lO0 parts by weight of polyester type polyurethane resin (product of Dainichiseika Color & Chemicals MFG. Co., LTD) was uniformly dissolved in 500 parts by weight of a mixed solvent of MEK and DMF (weight ratio of MED/DMF:

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z 10/90). 50 parts by weight of carbon black (average particle diameter: ~0 m~) and 30 parts by weight of graphite particle (average particle diameter: 8.8 ~m) were added and dispersed in the polyurethane solution. The solution had a viscosity of 80 poise at 30DC, as measured by B type viscometer.

While the above-prepared dipping solution was stirred, two-strand twisted polyester spun yarn of 20-count was dipped in and passed through the solution at a rate of 10 m/min at 20C, and the deposited amount of the solution was adjusted through dies of orifice size 0.6 mm. The dies were of stainless steel and of a type capable of being divided into two in setting the yarn thereon.

Thereafter, the dispersion deposited on the yarn was coagulated by continuously being passed through a coagulation bath of DMF aqueous solution (weight ratio of DMF/water: 2/98) maintained at 20C, and the solvent was sufficiently removed by being passed through a solvent-removing bath of water.

The yarn was dried by a Nelson type drying roller maintained at 120C.

The fibrous heating element produced by above method had a diameter of 0.5 mm, and an electric resistance value per length of 16 kQ/m.
Example 7 With Sample No. 4 prepared in Example 1 and ~-count polyester spun yarn used for woofs, and polyester filament (150D-50fil) and tin-plated copper wires (0.1 mm) for warps, a plain woven fabric was produced according to the normal method. In the above, the fibrous heating element of Sample No. 4 was woven in one in every 3 woofs of the count-4 polyester spun yarn. Also, the tin-plated copper wires were disposed in a number of 20 warps at each of the sides of the fabric, inside of edges of the warps, to form wire electrodes. The distance between the two electrodes was set to be 10 cm.

To the portion of wire electrodes of the woven fabric obtained above, polyethylene having 3.7 g/lOmin for its melt index and 0.923 g/cm3 for its density, was supplied molten at 310C through a nozzle slit of a melt extrusion laminator, and at the same time, both faces of the fabric were covered with a polyester film of a thickness of 25 ~m, followed by an application of pressure of 10 kg/cm to the fabric by water cooled rollers maintained at 30C, to form an insulating film and obtain a fabric heating element of 20 cm in length and 11 cm in width.

The above-obtained fabric heating element was rendered useable as a heater by connecting lead wires thereto.

The fabric heating element had a resistance value of 14n and was pliable and capable of being sewn like ordinary fabrics in general.

As can be seen from the results of the above Examples, the fibrous heating element is effectively useful as a heat generating element in a variety of goods such as (1) in the field of winter clothes such as outerwear for riders, fishermen, divers and so forth, innerwear, various workers' clothing, underwear, and so forth; (2) in the field of cold-prevention furnishings and bedding such as carpets, blankets, lap robes, seating material for railway passenger cars and automobiles, and other heating members and parts; (3) in the medical field such as medical-care supporters, stomach bands, warming mats and sheets and so forth; (4) in the field of house.hold heating goods such as gloves, shoes, socks, cushions and so forth;

(5) in the field of heating construction material such as flooring, walls, floor warmers and so forth; (6) in the field of electrical appliances such as various electrical instruments and appliances, heatlng members for various meters and so forth; and (7) in the field of agriculture and civil engineering such as bed warming sheets, maturing sheets and so forth.

Claims

1. A fibrous heating element comprising a core fiber consisting of a synthetic fiber coated with at least two layers of a polyurethane resin containing 30 to 100 parts by weight of at least one kind of carbon particles dispersed therein, based on 100 parts by weight of the polyurethane resin, and having an electrical resistivity of 1 to 100 k.OMEGA.
per meter.

2. A fibrous heating element as claimed in claim 1, wherein the core fiber is a spun yarn, a double-structured yarn or a textured yarn.

3. A fibrous heating element as claimed in claim 1 or 2, wherein the core fiber has a triangular, Y-shaped, T-shaped, cruciform-shaped, star-shaped, wedge-shaped, U-shaped, c-shaped, flat-shaped or concavoconvex-shaped cross-section.

4. A fibrous heating element as claimed in claim 1 or 2, wherein the synthetic fiber has a definite melting point.

5. A fibrous heating element as claimed in claim 1 or 2, wherein the synthetic fiber has an indefinite melting point.

6. A fibrous heating element as claimed in claim 1 or 2, wherein said at least two layers contain air bubbles.

7. A fibrous heating element as claimed in claim 1 or 2, wherein the polyurethane resin is cross-linked.

8. A method for the production of a fibrous heating element which comprises the steps of:
dipping a core fiber in a solution of a polyurethane resin containing 30 to 100 parts by weight of at least one kind of carbon black therein per 100 parts by weight of the polyurethane resin to deposit solution on the core fiber, solidifying the solution deposited on the core fiber, and repeating the steps of dipping the core fiber in the solution and solidifying the solution deposited on the core fiber at least one.

9. A method as claimed in claim 8, wherein the solidifying step comprises drying the solution deposited on the core fiber.

10. A method as claimed in claim 8, wherein the solidifying step comprises coagulating the solution deposited on the core fiber.

11. A method as claimed in claim 8, 9 or 10 wherein the polyurethane resin is cross-linked after it has been deposited on the core fiber.

12. A fabric heating element comprising fibrous heating elements and wire electrodes, wherein each of the fibrous heating elements comprises a core fiber coated with one or more electric conductive layers consisting of a polyurethane resin having carbon particles dispersed therein.

13. A fabric heating element as claimed in claim 12, wherein the core fiber is a spun yarn, a double-structured yarn, a multi-filament yarn or a textured yarn.

14. A fabric heating element as claimed in claim 12, wherein the fibrous heating elements and the wire electrodes are crossed.

15. A fabric heating element as claimed in claim 13, wherein the fibrous heating elements and the wire electrodes are crossed.

16. A fabric heating element as claimed in claim 12, 13, 14 or 15, which is formed by weaving.

17. A fabric heating element as claimed in claim 12, 13, 14 and 15, which is formed by knitting.

18. A fabric heating element as claimed in claim 16, wherein the wire electrodes are woven in as warps, which are fastened by a tangling yarn comprising a heat-shrinkable fiber.

19. A fabric heating element as claimed in claim 18, wherein the heat shrinkable fiber is shrunk after weaving.