FLEXIBLE THERMOSENSITIVE WIRE RESISTANT TO A DRY-CLEANING SOLVENT

Abstract: A flexible thermosensitive wire which comprises a core strand, a pair of electrodes one of which is spirally wound around the core strand, a thermosensitive polymer layer formed between the paired electrodes, a barrier layer for a dry-cleaning solvent formed around the thermosensitive polymer layer having the other electrode spirally wound therearound, and an outermost moisture-permeable layer. The wire is adapted for use in electric warming devices in which because of the quick moisture release of the wire, a normal thermosensitive characteristic is quickly shown in practical applications. The electric warming devices can be dry cleaned without deterioration of the wire.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to flexible thermosensitive wires which are adapted for use in electrically heated warming devices such as electric blankets, electric sheets or bedcovers. Description of the Prior Art Since electric blankets and the like electric personal warming devices are used in direct contact with the skin, there has been a high demand for laundering. To meet the demand, washable electric blankets have been developed, which make use of flexible thermosensitive wires using thermosensitive polymer materials such as polyamide compositions of low moisture absorption. In order for one to subject an electric blanket to dry cleaning, the heater wire has to be resistant to solvents. US-A-2 941 176 discloses a heater wire which is used in electrically heated fabrics, said wire comprising an outer protective layer of an extruded insulating composition containing from 20 to 40 percent by weight of a high molecular weight monomeric ester plasticizer, i.e. pentaerythritol ester, said plasticizer being used to prevent leaching out when the fabrics are subjected to dry cleaning. However, this wire has not been found satisfactory with respect to the solvent resistance. US-A-4 698 488 (which is based on JP-A-60-089901) discloses a flexible thermosensitive wire in which a thermosensitive polymer composition layer formed between at least one pair of electrodes is covered with a jacket. At least a part of the jacket is composed of a barrier polymer material which is resistant or impermeable to dry-cleaning solvents, e.g. of an ethylene-vinylalcohol copolymer, a polyamide or a highly crystalline polyolefin. The jacket may have a multi-layered structure including at least one barrier layer made of such a barrier polymer material. The at least one barrier layer of the multi-layered jacket is preferably forming the inside of the jacket and is covered with at least one layer forming an insoluble outer jacket which may be composed of an elastomer composition such as a polyvinyl chloride soft copolymer, e.g. ethylene-vinyl chloride copolymer, or a polyvinyl chloride polymer blend. EP-A2-0 125 913 discloses a flexible heating wire which comprises a core, a first conductive body, a second conductive body and a thermally fusible electrically insulative body arranged such that first and second conductive bodies will be brought into electric contact with each other when the thermally fusible electrically insulative body is thermally fused, a third conductive body and a heating body having a positive temperature coefficient and being held in electric contact with at least one of said first and second conductive bodies and the third conductive body. In this arrangement, the first, second and third conductive bodies serve as electrodes. The PTC heating body which may be made of polyamide contains a particulate conductive material such as carbon black. Polyamide containing a particulate conductive material would undergo considerable swelling by the action of a dry-cleaning solvent. Thermosensitive polymer materials of a low moisture absorption are used in these flexible thermosensitive wires, but the polymeric barrier materials or non-dissolving polymer materials are also used to cover the thermosensitive polymer material. Accordingly, release of moisture from the thermosensitive polymer material or a material inside the thermosensitive polymer material is prevented. This involves the problem that the recovery of the thermosensitive characteristic becomes very slow. When such a wire is applied as an electric blanket, the temperature of the blanket cannot be precisely controlled during use within a short period of time. SUMMARY OF THE INVENTION It is the object of the invention to provide a flexible thermosensitive wire which is adapted for use in electrically heated warming devices such as an electric blanket, and in which, while a thermosensitive polymer material is best utilized, the wire has a high resistance to a dry-cleaning solvent when applied to electrically heated warming devices without preventing the release of moisture from the thermosensitive polymer material or from a material inside the thermosensitive polymer material. According to the invention, said object is achieved by a flexible thermosensitive wire resistant to a dry-cleaning solvent which comprises a core strand (1), a pair of electrodes (2, 3) in a spaced relation with each other, a thermosensitive polymer layer (4) formed between the paired electrodes (2, 3) and completely covering the core strand (1), a solvent barrier layer (5) comprising a hydrophilic group-containing polymer, which is resistant to a dry-cleaning solvent, said solvent barrier layer (5) being formed to cover the thermosensitive polymer layer (4) and one of the paired electrodes (2, 3) formed on the thermosensitive polymer layer (4), and an outermost layer (6) of a soft polymer material resistant to a dry-cleaning solvent, said outermost layer (6) being formed to cover the solvent barrier layer (5), characterized in that said solvent barrier layer (5) is permeable to moisture and said outermost layer (6) is a moisture-permeable layer (6) which has a better vapor permeability than the solvent barrier layer (5), and in that a separation layer (10, 11) is provided between said solvent barrier layer (5) and said thermosensitive polymer layer (4) and one of the paired electrodes (2, 3) wound around said thermosensitive polymer layer (4) to prevent a direct contact between said solvent barrier layer (5) and said thermosensitive polymer layer (4). In accordance with a more specific embodiment of the invention, the pair of electrodes (2, 3) comprises a first electrode (2) spirally or helically wound around the core strand (1), and a second electrode (3) spirally wound around the thermosensitive polymer layer (4) of a polyamide composition, the thermosensitive polymer layer (4) covering the core strand (1) and the first electrode (2), and the solvent barrier layer (5) covering the thermosensitive polymer layer (4) and the second electrode (3). The electrodes used in the above embodiments may usually take a ribbon form. The flexible thermosensitive wire of the invention has good thermal stability of the thermosensitive polymer layer in relation to time and, when applied to, for example, an electric blanket, can stably maintain the temperature of the blanket, which is thus comfortable during use. When such a blanket becomes dirty, it can be dry-cleaned. This is very advantageous from the practical point of view. More particularly, the wire according to the invention can quickly release moisture. Even when it takes up moisture, the thermosensitive characteristic returns to an original level within a short time. In addition, the wire is resistant to a dry-cleaning solvent and only a very small degree in variation of the thermosensitive characteristic is experienced. The wire which is clean dried has held its flexibility. Because the separation layer is further formed, the thermosensitive polymer layer is prevented from deterioration with time and the thermosensitive wire has a very excellent thermal stability with time with respect to the thermosensitive characteristic. Moreover, the wire may be readily worked at a terminal thereof for use as a lead wire by elimination of the outer layers because of the presence of the separation layer. This gives a great merit in terminal mold processings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic illustrative view of a flexible thermosensitive wire according to one embodiment of the invention; Fig. 2 is a schematic illustrative view, similar to Fig. 1, of another embodiment of the invention; DETAILED DESCRIPTION AND EMBODIMENTS OF THE INVENTION Reference is now made to the accompanying Figures 1 and 2, which show a flexible thermosensitive wire according to the invention. The wire comprises an insulative core strand 1 made, for example, of a polyester resin or other materials ordinarily used for these purposes. Around the core strand 1 is spirally or helically wound an electrode 2 which may take a ribbon form as shown. The electrode 2 is completely covered with a thermosensitive polymer layer 4, which is in turn helically wound with an electrode 3 as shown. The electrodes 2 and 3 are made, for example, of copper or copper alloys. The thermosensitive polymer layer 4 has a substantially uniform thickness of, for example, 0.1 to 0.5 mm along the length thereof and has thus a generally cylindrical cross-sectional configuration. This layer 4 is required to have a thermistor characteristic and/or a fusing function and is made, in most cases, of a polyamide composition in order to utilize the thermosensitive characteristic of hydrogen bonds of the amido groups. The polyamide composition should preferably be formulated to have thermistor characteristics as desired. Such a composition comprises, for example, 50 to 90 wt% of a polyamide resin and 50 to 10 wt% of a thermoplastic phenolic resin. However, the electrodes 2,3 and the thermosensitive polymer layer 4 are not critical with respect to the type of material or composition and one may employ any known materials for these electrodes 2,3 and the layer 4 in the practice of the invention. For instance, the thermosensitive polymer layer may be formed of a polymer composition whose electric resistance, electrostatic capacity or impedance is of the negative or positive temperature dependence or may be formed of a crystalline polymer composition having a temperature-fusing function. If the thermosensitive polymer layer 4 were brought into intimate contact with the solvent barrier layer 5 to be described later, gradual migration of the resins might take place. This would cause the thermosensitive characteristic of the layer 4 to vary with time. If the solvent barrier layer 5 is formed of a polyamide composition and the thermosensitive polymer layer 4 is also made of a polyamide composition, the migration is more liable to occur. To avoid this, a separation layer is formed, as 10 in Fig. 1 or 11 in Fig. 2, between the thermosensitive polymer layer 4 helically wound with the electrode 3 and the solvent barrier layer 5. In Fig. 1, the separation layer 10 is spirally wound over the thermosensitive polymer layer 4 helically wound with the electrode 3 without leaving a space between any adjacent spiral turns for complete coverage. The separation layer 11 of Fig. 2 is to wrap the layer 4 helically wound with the electrode 3 as shown. The separation layer 10 or 11 is made, for example, of a polyester or polyolefin film. By the provision of the separation layer 10 or 11, the migration phenomenon can be fully prevented, by which the thermosensitive characteristic of the layer 4 is reliably kept. The separation layer 10 or 11 is then covered with a solvent barrier layer 5 which is resistant to a dry-cleaning solvent, and with a moisture-permeable outermost layer 6, both formed such as by extrusion or other techniques. The hydrophilic group-containing polymers used as the solvent barrier layer 5 have all high crystallinity and exhibit a good resistance to a dry-cleaning solvent. However, the highly crystalline polymers have a greater modulus of elasticity than ordinary soft polymer materials. If the solvent barrier layer 5 were provided as an outermost layer of the flexible thermosensitive wire, the flexibility of the wire would lower, making it rather unfit for practical applications. Accordingly, the solvent barrier layer 5 is applied over the thermosensitive polymer layer 4, the electrode 3, and the separation layer 10 or 11, and over the solvent barrier layer 5 the moisture-permeable outermost layers 6 is applied. The polymers which contain hydrophilic groups, such as a hydroxyl group, an amido group and/or a carboxyl group, have high affinity for water molecules and have a great moisture permeability. Such polymers have thus both a good resistance to a dry-cleaning solvent and a good moisture permeability. Accordingly, hydrophilic group-containing polymers such as ethylene-vinyl alcohol copolymers or polyamides are used as the solvent barrier layer 5. Of ethylene-vinyl alcohol copolymers, random or alternating copolymers are most preferred because of the better moisture permeability. On the other hand, a number of polyamides are known including nylon homopolymers such as nylon 6, nylon 11, and nylon 12, and copolymers such as polyether-amides, polyester-amides, and N-substituted polyamides. Of these, nylon homopolymers have generally high crystallinity and, thus, exhibit a good resistance to a dry-cleaning solvent. However, when the content of the amido groups such as in nylon 6 increases, the modulus of elasticity undesirably increases, resulting in a lowering of flexibility of the resultant flexible thermosensitive wire. In view of the above, most preferable polyamides are nylon 11 and nylon 12. The solvent barrier layer 5 has generally a thickness of from 0.05 to 0.2 mm in order to ensure the resistance to a dry-cleaning solvent and allow a suitable degree of moisture permeability. The moisture-permeable outermost layer 6 covering the solvent barrier layer 5 is made of soft polymer materials or compositions which exhibit a moisture permeability equal to or better than the hydrophilic group-containing polymers of the solvent barrier layer 5 and a good solvent resistance. A great number of soft polymer materials or compositions are known, of which polyvinyl chloride compositions are preferred. Because of the presence of vinyl chloride units, it is easy to impart a flame resistance to the composition, e.g. when ordinary flame retardants such as antimony oxide are added to a polyvinyl chloride composition, a flame resistance expressed by a lowest oxygen index not less than 25 is readily imparted. Accordingly, such a polyvinyl chloride composition is suitable for use as a material for the outermost layer or jacket 6 of this type of flexible thermosensitive wire. Polyvinyl chloride is easily softened by addition of other polymers or plasticizers. Other polymers for blending with the polyvinyl chloride should preferably have a great moisture permeability and are, for example, those polymers which have urethane groups or ester groups having relatively good affinity for water molecules. Examples of such polymers include polyurethanes and vinyl chloride copolymers such as vinyl chloride-urethane copolymers, vinyl chloride-acrylic copolymers and vinyl chloride-vinyl acetate copolymers. Of these, polyurethanes and vinyl chloride-urethane copolymers are preferred sine they are very soft and have very high moisture permeability with good resistances to solvent and cold temperature. Compositions which comprise blends of polyvinyl chloride and polyurethanes or vinyl chloride-urethane copolymers have a high moisture permeability and a good solvent resistance with good cold temperature and flame resistances. The term "solvent resistance" used herein means little or no solubility in dry-cleaning solvents. Only one drawback involved in polyurethanes or vinyl chloride copolymers is a low heat resistance. In general, the heat resistance of a plasticized polyvinyl chloride composition greatly varies depending on the type of plasticizer. Heat-resistant plasticizers include e.g. polyesters, trimellitates, pentaerythritol esters, and epoxy compounds. Since polyester plasticizers have ester groups, they have high affinity for water molecules. In addition, ester compounds are readily polymerized, so that when the compounds are polymerized to have an average molecular weight not less than 6000, the resultant polyesters exhibit a good solvent resistance. The polyester plasticizers may be classified into phthalate plasticizers, adipate plasticizers and sebacate plasticizers, among which the sebacate plasticizers are preferred. This is because sebacic acid is a long-chain dibasic acid with a long distance between the polar groups of the ester bonds and a great degree of freedom of the molecules, so that this type of plasticizer is most compatible with polyvinyl chloride. If polyvinyl chloride and a polyester plasticizer having an average molecular weight not less than 6000 are mixed, the resulting composition exhibits a high vapor permeability and a good solvent resistance with good heat and flame resistances. However, the plasticizer is so high in molecular weight that the composition has not always a satisfactory cold resistance. A preferable polyvinyl chloride composition for the outermost layer 6 comprises 100 parts by weight of polyvinyl chloride, 30 to 100 parts by weight of a polyester plasticizer having an average molecular weight not less than 6000, and 20 to 80 parts by weight of a polyurethane or a vinyl chloride copolymer. The vinyl chloride copolymer includes, for example, a vinyl chloride-urethane copolymer, a vinyl chloride-acrylic copolymer, a vinyl chloride-vinyl acetate copolymer and mixtures thereof. This composition has a high moisture permeability, an excellent solvent resistance and good heat, cold and flame resistances and is thus suitable for use as the flexible, moisture-permeable outermost layer. If the amount of the polyester plasticizer is less than 30 parts by weight, the moisture permeability and heat resistance become worse. Over 100 parts by weight, the compatibility becomes poor, causing a bleedout phenomenon. On the other hand, when the amount of the polyurethane or vinyl chloride copolymer is less than 20 parts by weight, the moisture permeability and flexibility deteriorate. Over 80 parts, the heat resistance becomes poor. In order to further improve the mechanical strength and heat resistance of the polyvinyl chloride composition, it is preferred that the polyvinyl chloride used should have an average degree of polymerization of not less than 1800. Since the flexible thermosensitive wire of the invention is adapted for use in electric blankets or sheets where cloths or woven fabrics are heated during use, it is desirable that the mechanical strength be high with high heat and flame resistances. These characteristic properties can all be imparted to the wire when using the above polyvinyl chloride composition. As is known in the art, polyvinyl chloride grains consisting of granules or particles are each covered with a thin skin on the surface thereof. These grains do not separate into particles even when a plasticizer is added to the polyvinyl chloride, tending to cause fisheyes. To avoid this, skinless grains of polyvinyl chloride are conveniently used. The skinless grains have been prepared according to a specific known manner of polymerization. These grains have improved compatibility with plasticizers and good dispersability. Thus, the skinless grains are preferred for use in the polyvinyl chloride composition used for the outermost layer. The outermost layer has generally a thickness of 0.2 to 0.6 mm in order to fully show its characteristic properties. The flexible thermosensitive wire according to the invention may be used, for example, as a temperature sensor when the built-in electrodes are used as signal wires, or as a thermosensitive heater when at least one electrode is used also as a heating conductor. In general, dry cleaning takes about 30 minutes in one process, in which tetrachloroethylene or petroleum solvents are used as a dry-cleaning solvent. The flexible thermosensitive wire of the invention can stand use upon exposure to the dry-cleaning solvent within the above time, within which the thermosensitive characteristic does rarely vary while keeping the flexiblity of the wire without dissolution of a plasticizer in the dry-cleaning solvent. The present invention is more particularly described by way of examples. Example 1 In this example, a flexible thermosensitive wire having such a construction as shown in Fig. 1 was fabricated. The wire had a polyester core 1, a pair of ribbon electrodes 2,3 of a copper alloy, a thermosensitive polymer layer 4 of a low moisture absorption made of a nylon 12 composition incorporated with a phenolic resin material, a polyester separation layer 10, a 0.15 mm thick solvent barrier layer 5 of nylon 12 and a 0.4 mm thick outermost layer 6. The outermost layer 6 was made of a composition comprised of 100 parts by weight of polyvinyl chloride, 30 parts by weight of polyurethane, 50 part by weight of a polyester plasticizer having an average molecular weight of 8000 (Polycizer P-202, produced by Dainippon Ink Chem. Co., Ltd.), 3 parts by weight of a Ba-Zn thermal stabilizer, and 15 parts by weight of a mixture of antimony oxide and calcium carbonate. The flexible thermosensitive wire was subjected to a heat-resistant test. The test was effected in such a way that the wire was dried at 100 o C for 10 hours and subjected to measurement of impedance (Z o ). Thereafter, the wire was allowed to stand at 100 o C for 500 hours and then subjected again to measurement of impedance (Z). The variation of the impedance of ΔZ H = Z/Z o was used for the evaluation. The results are shown in Table 1. Examples 2 and 3 The general procedure of Example 1 was repeated except that the materials for the solvent barrier layer 5 or the separation layer 10 were changed as indicated in Table 1 and the manner of winding of the separation layer was changed in Example 2. The results are also shown in Table 1. For reference, the above procedure was repeated except for the modifications indicated in Table 1. The above results reveal that the wires having the separation layer are improved over the separation layer-free wires. The separation layer is advantageous in that when the wire terminal is worked for electric connection, this can be done readily.

Claims: A flexible thermosensitive wire resistant to a dry-cleaning solvent which comprises a core strand (1), a pair of electrodes (2, 3) in a spaced relation with each other, a thermosensitive polymer layer (4) formed between the paired electrodes (2, 3) and covering the core strand (1), a solvent barrier layer (5) comprising a hydrophilic group-containing polymer, which is resistant to a dry-cleaning solvent, said solvent barrier layer (5) being formed to cover the thermosensitive polymer layer (4) and one of the paired electrodes (2, 3) formed on the thermosensitive polymer layer (4), and an outermost layer (6) of a soft polymer material resistant to a dry-cleaning solvent, said outermost layer (6) being formed to cover the solvent barrier layer (5), characterized in that said solvent barrier layer (5) is permeable to moisture and said outermost layer (6) is a moisture-permeable layer (6) which has a better vapor permeability than the solvent barrier layer (5), and in that a separation layer (10, 11) is provided between said solvent barrier layer (5) and said thermosensitive polymer layer (4) and one of the paired electrodes (2, 3) wound around said thermosensitive polymer layer (4) to prevent a direct contact between said solvent barrier layer (5) and said thermosensitive polymer layer (4). A wire according to claim 1, characterized in that said pair of electrodes (2, 3) comprises a first electrode (2) spirally wound around said core strand (1) and a second electrode (3) spirally wound around the thermosensitive polymer layer (4) of a polyamide composition. A wire according to claim 1, wherein said hydrophilic group-containing polymer is a nylon homopolymer. A wire according to claim 3, wherein said nylon homopolymer is nylon 11 or nylon 12. A wire according to claim 1 or 2, wherein said moisture-permeable outermost layer (6) is a polyvinyl chloride composition. A wire according to claim 5, wherein said polyvinyl chloride composition comprises a mixture of 100 parts by weight of polyvinyl chloride, 30 to 100 parts by weight of a polyester plasticizer having an average molecular weight of not less than 6000, and 20 to 80 parts by weight of at least one member selected from vinyl chloride-urethane copolymer, vinyl chloride-acrylic copolymer, vinyl chloride-vinyl acetate copolymer and polyurethane. A wire according to claim 6, wherein said polyester plasticizer is a sebacate plasticizer. A wire according to claim 6, wherein said polyvinyl chloride composition further comprises a flame retardant so that it has a lowest oxygen index of not less than 25. A wire according to claim 6, wherein said polyvinyl chloride has an average degree of polymerization of not less than 1800. A wire according to claim 6, wherein said polyvinyl chloride is in the form of skinless grains. A wire according to claim 1, wherein said thermosensitive polymer layer (4) is made of a polyamide composition, said solvent barrier layer (5) is made of another polyamide composition and said separation layer (10, 11) is a polyester or polyolefin film. A wire according to claim 1, wherein said separation layer (10) is spirally wound over said thermosensitive polymer layer (4) without leaving a space between any adjacent turns. A wire according to claim 1, wherein said separation layer (10) completely wraps said thermosensitive polymer layer (4).

Inventors: HOSAKA, TOMIHARU, KISHIMOTO, YOSHIO
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1986
Main classification: H05B3
IPC codes: H05B3, H01B7