Electric wire

1. Field of the Invention

The present invention relates to an electric wire used in wiring in equipment such as electronic equipment or the like and, more particularly, to an electric wire attached to a terminal of an insulation displacement connector.

2. Related Background Art

An electric wire for an insulation displacement connector generally comprises a stranded conductor obtained by twisting a plurality of conductor wires and a covering layer covering the stranded conductor. The end portion of the electric wire is inserted under pressure between two contact portions of a terminal of an insulation displacement connector having a plurality of terminals and two contact portions of the terminal on the sides of the electric wire penetrate the covering layer and contact the stranded conductor and are electrically connected to the stranded conductor. The insulation displacement connector has a wedged portion called strain relieves so as not to allow removal of the electric wire once accommodated (see reference numeral 6 in FIG. 2 ).

Polyvinyl chloride (referred to as “PVC” hereinafter) or the like was conventionally used as the material constituting the covering layer of the above-described electric wire for insulation displacement connector because PVC is inexpensive and excellent in workability. However, PVC produces a toxic halogen gas during combustion and may generate very toxic dioxin during incineration for disposal. Also, the U.S. UL (Underwire Laboratories Inc.) standard requires that the electric wire should have a certain degree of fire retardancy. Therefore, a material which does not produce toxic gases such as a halogen gas or the like and has a certain degree of fire retardancy is required as the material constituting the covering layer of the electric wire.

Under these circumstances, for example, as described in Japanese Patent No. 2525982, a resin composition including a fire retardant such as aluminum hydroxide or magnesium hydroxide or the like in a polyolefin resin is irradiated with ionizing radiation, and the resultant material is used as the material for the covering layer.

The present inventors have found that use of the material disclosed in the prior-art reference as the covering layer of an electric wire for an insulation displacement connector suffers the following problem.

Namely, according to the findings of the present inventors, as shown in FIG. 1, when an electric wire 3 is inserted under pressure between two contact portions 2 of a terminal of an insulation displacement connector 1 to bring a conductor 4 in electric wire 3 into contact with two contact portions 2 of the terminal, a covering layer 5 tears to expose conductor 4 in electric wire 3 and this may cause the short-circuit in the connector. As shown in FIG. 2, covering layer 5 deforms and electric wire 3 cannot be properly mounted in connector 1 . Electric wire 3 once mounted may be removed.

It is an object of the present invention to provide an electric wire capable of sufficiently preventing removal of the electric wire from an insulation displacement connector and sufficiently preventing exposure of the conductor.

The present inventors have made extensive studies to solve the above problems. Namely, as to whether in electric wires having covering layers made of various materials, conductors are exposed (referred to as “core exposure” hereinafter), or the covering layers deform (referred to as “covering layer deformation” hereinafter) when electric wires for insulation displacement connectors are inserted under pressure between two contact portions of a terminal of insulation displacement connectors, the present inventors have conducted investigations by calculating the frequencies of “core exposure” and “covering layer deformation” occurring in the electric wires for evaluation. The relative relationship between this result and the 100% tensile modulus and elongation of the covering material was examined. The result is shown in FIG. 3 . In FIG. 3, when the frequency of “core exposure” or “covering layer deformation” is {fraction (1/20)} or less, the result is determined as “good” and indicated by “◯”. When the frequency is higher than {fraction (1/20)}, the result is determined as “poor” and indicated by “▴” for “covering layer deformation”, and “×” for “core exposure”. Also, in FIG. 3, G100 represents Y=7.8, while G200 represents E 1 =270−8.5×10 −6 ×Y. A stranded conductor whose conductor size is AWG26 (obtained by twisting seven tinned wires each having a diameter of 0.16 mm) was used as a conductor of an electric wire for insulation displacement connectors. Covering layer is obtained by covering a resin composition having a predetermined composition and extruded by an 50-mm diameter extruder on the stranded conductor to have a conductor outer diameter of 0.98 mm, and then irradiating an electron beam at predetermined doses to the resin composition. Also, a JST DA connector having 5 terminals arrayed at the pitch of 2 mm (manufactured by JST Mfg Co., Ltd) was used as an insulation displacement connector.

The present inventors have found that the above problems in prior art can be solved by using a covering material such that its 100% tensile modulus and elongation satisfy predetermined conditions, thus accomplishing the present invention.

Namely, the electric wire of the present invention comprises a conductor and a covering layer covering the conductor, the conductor being brought into contact with two contact portions of a terminal by inserting the electric wire under pressure between two contact portions of the terminal of an insulation displacement connector, wherein the covering layer is made of a covering material obtained by irradiating with ionizing radiation a resin composition containing an ethylene copolymer and a metal hydroxide surface-treated with a silane compound represented by:

(where R represents an alkyl group having an acrylic, methacrylic, or allyl group, a saturated alkyl group, a vinyl group, an epoxy group, an amino group, or a mercapto group; X1, X2, and X3 represent alkoxy or alkyl groups, respectively; and at least one of X1, X2, and X3 represents an alkoxy group), a 100% tensile modulus of the covering material is not less than 7.8 MPa, and the 100% tensile modulus and an elongation of the covering material satisfies the following relationship:

E 1 >270−8.5×10 −6 ×Y

(where E 1 is the elongation and Y is the 100% tensile modulus).

According to the present invention, in inserting under pressure the electric wire between two contact portions of the terminal of the insulation displacement connector, the electric wire can be reliably mounted in the insulation displacement connector without deforming the covering layer, and the removal of the electric wire once mounted can be prevented. Also, damage to the covering layer and exposure of the conductor can be sufficiently prevented.

The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.

FIG. 1 is a plan view showing a comparative example for explaining the effect of an electric wire of the present invention, and showing “core exposure” occurring in the electric wire;

FIG. 2 is a side view showing another comparative example for explaining the effect of the electric wire of the present invention, and showing “covering layer deformation” occurring in the electric wire;

FIG. 3 is a graph showing the relative relationship between 100% tensile modulus, elongation, and evaluation of insulation displacement workability of various electric wires;

FIG. 4 is a plan view showing the electric wire of the present invention mounted in an insulation displacement connector; and

FIG. 5 is a sectional view taken along the line V—V in FIG. 4 .

An electric wire according to the present invention will be described with reference to FIGS. 4 and 5 .

As shown in FIGS. 4 and 5, electric wires 14 of the present invention comprise a conductor 10 and a covering layer 12 covering conductor 10 . According to electric wire 14 of the present invention, when the end portion of electric wire 14 is inserted under pressure between two contact portions 16 of a terminal of an insulation displacement connector 18 and between strain relieves 20 , electric wire 14 can be reliably mounted in insulation displacement connector 18 without deforming covering layer 12 , and the removal of electric wire 14 once mounted can be prevented. Also, damage to covering layer 12 and exposure of conductor 10 can be sufficiently prevented.

Conductor 10 used in the present invention may be a single-wire conductor comprised of a single conductor wire or a stranded conductor obtained by twisting a plurality of conductor wires. When a stranded conductor is used, the number of wires is generally 7 to 43 and preferably 7. When the electric wire comprising a 7-wire stranded conductor is mounted in the terminal of the insulation displacement connector, the contact area between the conductor and the terminal can be increased, thereby further improving connection reliability. The conductor wire is not particularly limited if it is conductive. Examples of the conductor wire are an annealed copper wire, a hard-drawn copper wire, an annealed copper tinned wire, a copper-tin alloy wire, a copper-plated steel wire, and a noble metal wire.

The diameter of conductor 10 is appropriately selected depending on the terminal pitch of insulation displacement connector 18 used. Namely, an electric wire comprising a conductor having a conductor size of AWG32 or AWG30 is used for an insulation displacement connector having a terminal pitch of 1 mm; an electric wire comprising a conductor having a conductor size of AWG30 or AWG28 is used for an insulation displacement connector having a terminal pitch of 1.5 mm; an electric wire comprising a conductor having a conductor size of AWG28 or AWG26 is used for an insulation displacement connector having a terminal pitch of 2.0 mm; an electric wire comprising a conductor having a conductor size of AWG28, AWG26 or AWG24 is used for an insulation displacement connector having a terminal pitch of 2.5 mm; and an electric wire comprising a conductor having a conductor size of AWG26, AWG24, AWG22, AWG20, or AWG18 is used for an insulation displacement connector having a terminal pitch of 3.96 mm.

Covering layer 12 used in the present invention is made of a covering material. The 100% tensile modulus of this covering material is 7.8 MPa or more. Here, the 100% tensile modulus means a tensile strength at the 100% elongation when the covering layer is pulled at an inter-chuck distance of 50 mm, a distance of 20 mm between two gage marks, and a stress rate of 500 mm/min using a tensile test machine(“tensilon” manufactured by Orientec Inc.). When the 100% tensile modulus of this covering material is less than 7.8 MPa, covering layer 12 deforms when the end portion of electric wire 14 is mounted in insulation displacement connector 18 because the covering material is soft. The lower limit of the 100% tensile modulus is preferably 9 MPa, and more preferably 10 MPa. On the other hand, the upper limit of the 100% tensile modulus of the covering material is preferably 50 MPa.

The 100% tensile modulus (Pa) and the elongation (%) of the covering material used in the present invention satisfy the following relational expression:

E 1 >270−8.5×10 −6 ×Y   (1)

(where E 1 is the elongation and Y is the 100% tensile modulus). Here, the elongation means an elongation when the covering layer 12 is pulled until it breaks using the tensile test machine described above. When the elongation and 100% tensile modulus of the covering material do not satisfy the above expression (1), covering layer 12 cracks to expose the conductor.

A covering material satisfying the above relationship is obtained by irradiating with ionizing radiation a resin composition containing an ethylene copolymer and a metal hydroxide surface-treated with a silane compound represented by the following general formura:

(wherein R represents an alkyl group having an acrylic, methacrylic, or allyl group, a saturated alkyl group, a vinyl group, an epoxy group, an amino group, or a mercapto group; X1, X2, and X3 represent alkoxy or alkyl groups, respectively; and at least one of X1, X2, and X3 represents an alkoxy group).

In the present invention, the ethylene copolymer means a copolymer of ethylene and other monomer(s). Examples of the ethylene copolymer are an ethylene-vinyl acetate copolymer, an ethylene-ethyl acrylate copolymer, an ethylene-methyl acrylate copolymer, an ethylene-α-olefin copolymer, or a mixture thereof.

Examples of the metal hydroxide contained in the covering material of the present invention are magnesium hydroxide, aluminum hydroxide or the like.

In the resin composition used in the present invention, 90 to 250 parts by weight of a metal hydroxide are preferably added to 100 parts by weight of the ethylene copolymer. When the added amount of the metal hydroxide is less than 90 parts by weight, fire retardancy of the covering layer tends to be insufficient. When the added amount of the metal hydroxide is more than 250 parts by weight, mechanical properties of the covering layer tends to degrade.

The silane compound represented by the above general formula is a material generally called a silane coupling agent and is used for surface-treating a metal hydroxide. A metal hydroxide is surface-treated with the above silane compound so as to improve the mechanical characteristics of the covering layer. The surface treatment of the metal hydroxide with the silane compound can be performed by a known method. For example, a predetermined amount of a silane compound is dispersed in water so as to obtain a concentration of 0.1% to several %, and the pH of the aqueous solution is adjusted as required, thereafter a predetermined amount of a metal hydroxide is dipped in the aqueous solution, and the resultant solution is stirred to obtain a slurry. The slurry is filtered, heated, and dried to obtain metal hydroxide surface-treated with the silane compound. Also, the metal hydroxide can be surface-treated by mixing the metal hydroxide, which has not been surface-treated, with ethylene copolymer and a silane compound using a roll mixer or the like (integral blending method).

Examples of the silane compound are alkoxysilanes (e.g., methyltrimethoxysilane, dimethyltrimethoxysilane, trimethylmethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, and ethyltriethoxysilane, butyltrimethoxysilane), acrylsilanes (e.g., γ-methacryloxypropyltrimethoxysilane), vinyl silanes (e.g., vinyltri(βmethoxyethoxy)silane, vinyltriethoxysilane and vinyltrimethoxysilane), epoxy silanes (e.g., β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, γ-glycidoxypropyltrimethoxysilane, and γ-glycidoxypropylmethyldiethoxysilane), aminosilanes (e.g., N-β(aminoethyl)γ-aminopropyltrimethoxysilane, N-β(aminoethyl)γ-aminopropylmethyldimethoxysilane, γ-aminopropyltriethoxysilane, N-phenyl-γ-aminopropyltrimethoxysilane), and mercapto silanes (e.g., γ-mercaptopropyltrimethoxysilane).

The added amount of the silane compound in the resin composition is preferably 0.2 to 2.0 parts by weight to 100 parts by weight of a metal hydroxide. When the added amount of the silane compound is less than 0.2 parts by weight, mechanical properties of the covering layer tend to be insufficient. When the added amount of the silane compound exceeds 2.0 parts by weight, the upper portion of the covering layer tends to crack to expose the conductor when the end portion of the electric wire is inserted under pressure between two contact portions of the terminal of the insulation displacement connector.

The above resin composition may contain other polymer different from the ethylene copolymer, for example, polyethylene (e.g., high-density polyethylene), polypropylene, ethylene-propylene rubber. The resin composition may contain the above silane compound singly together with the metal hydroxide surface-treated with a silane compound. In addition, the resin composition may contain various thermal stabilizers, ultraviolet absorbing agents, lubricants, antioxidants, coloring agents, foaming agents, working stabilizers, organic or inorganic fillers or the like as required. To increase the crosslinking efficiency in irradiation with ionizing radiation, the resin composition may contain a crosslinking assistant such as trimethylolpropantrimethacrylate, pentaerythritoltriacrylate, ethylene glycol dimethacrylate, triallylcyanurate, triallylisocyanurate or the like.

The above resin composition is melted and kneaded using a monoaxial extruder, multi-axial extruder, Banbury mixer, roll, kneader, or the like and is extruded in the form of a tube to cover the conductor.

A γ-ray or electron beam is used as the ionizing radiation for irradiating the resin composition. The 100% tensile modulus and elongation of the resultant covering material can be controlled to satisfy the above expression (1) by adjusting a dose of the ionizing radiation.

Here, the dose of the ionizing radiation is different depending on the hardness of the resin composition and the added amount of the silane compound to the resin composition, but it is preferably within the range of 20 to 130 kGy. If the dose is less than 20 kGy, the holding force of the terminal of the insulation displacement connector for the electric wire tends to deteriorate upon use over a long period of time because three-dimensional crosslinking is insufficient in the resultant covering material, thereby the covering layer easily deforms. When the dose exceeds 130 kGy, the covering layer tends to easily crack to expose the conductor.

Here, when the added amount of silane compound to metal hydroxide is expressed by A (parts by weight), the resin composition is preferably irradiated with the ionizing radiation at a dose not more than a dose represented by the following expression (2) and within the above-described range of dose of ionizing radiation:

(180−100× A )(kGy)  (2)

The reason is: although the higher the dose of the ionizing radiation on the resin composition and the higher the added amount of the silane compound, the easier the covering layer cracks when the electric wire is mounted in the insulation displacement connector, generation of cracks in the covering layer can be sufficiently prevented at the time of mounting the electric wire to the insulation displacement connector when the resin composition is irradiated with the ionizing radiation at the dose represented by the above expression (2).

In the covering material obtained by irradiating the resin composition with the ionizing radiation, the gel fraction G of a portion excluding inorganic substances including the metal hydride from the covering material is preferably 55% to 85%.

When the gel fraction G is less than 55%, the holding force of the terminal of the insulation displacement connector tends to deteriorate for a long period of time because three-dimensional crosslinking is insufficient in the covering material, the covering layer easily deforms. When the gel fraction G exceeds 85%, the covering layer tends to easily crack to expose the conductor.

The gel fraction is an index representing the degree of crosslinking. The gel fraction means the ratio of gel (insoluble polymer chains) contained in a portion excluding inorganic substances including a metal hydride from the covering material insoluble in the solvent such as xylene or the like. The gel fraction G (%) is represented by the following expression:

G (%)=( G′−M )×100/(100 ×M )

(where G′ is the apparent gel fraction (%) and M is the weight (%) of the inorganic substances such as a metal hydroxide or the like in the covering material. The apparent gel fraction G′ (%) can be represented by the following expression:

G ′(%)=(weight of covering material after extracting solvent)×100/(weight of covering material before solvent extraction)).

The insulation displacement connector to which the electric wire of the present invention is mounted is not particularly limited and may be any kind of connector, for example, a variety of insulation displacement connectors manufactured by JST Mfg Co., Ltd (JST) and Tyco electronics AMP K.K. (AMP) can be used as an insulation displacement connector. The insulation displacement connectors have 2 to 16 terminals, and an insulation displacement connector with an appropriate number of terminals can be used. Further, the terminal pitch of the insulation displacement connectors ranges from 1.0 mm to 3.96 mm. An insulation displacement connector having an appropriate terminal pitch is used depending on the type of the electric wire used.

The contents of the present invention will be described in more detail by way of its examples.