PROCESS FOR PRODUCING AN INSULATED TWISTED ELECTRIC WIRE

Abstract: A process for producing an insulating twisted electric wire having a reduced overall diameter with an insulating coat­ ing of high quality. The insulating coating has a thickness in a range of 3 to 100% of the radius of a smallest circle circum­ scribing the assembled set of twisted conductors. The insulat­ ing coating is formed by two or more cycles of coating and cur­ ing an insulating paint. Preferably, a bottom layer of the insulat­ ing coating is formed by applying a heat-curable insulating paint with a felt applicator or with a roller coating device. The heat-curable insulating paint should have a viscosity of not more than about 300 cps at 30°C.

Description: The present invention relates to a process for producing twisted insulated electric wires used for wiring in a variety of electronic devices. More particularly, the invention relates to a process for producing a twisted wire having a plurality of conductors with a thin insulating coating provided by applying insulating paint to the conductors and baking the applied layer. Insulated wires of this type used for wiring in a large variety of electronic devices are conventionally produced by extruding a covering of insulating material over the twisted conductors. A conventional insulated electric wire of a similar type is shown, for example in US-A-2,438,956. Such insulated wires have been used either independently or as conductors for shielded wires, coaxial cables or flat cables. With the recent tendency to smaller and lighter electronic devices, considerable efforts are being made to further reduce the cross-sectional size of insulated wires, shielded wires and cables. One way of doing this is to reduce the thickness of the insulating coating. It is very difficult, however, to form an adequately thin insulation coating with current extrusion techniques. An alternative that has been proposed is to provide a thin insulating coating by multiple applications and curings of insulating paint. In actual operation, however, problems such as blistering of the coating or entrapment of air bubbles within the coating occur during the steps of applying the insulating paint to the twisted conductors and curing the applied paint. More specifically, when the insulating paint applied to twisted conductors is cured, air left in gaps between twisted conductors, as shown in Fig. 1, will expand as a result of heating to cause blistering in the surface of the coating. This problem can be significantly reduced by baking the applied paint at lower temperatures, for instance, 150°C or below, but then the coating finally obtained does not harden sufficiently to provide a reliable twisted insulated wire. In order to solve the problems described above, the present inventors have conducted various studies to find an effective method to produce twisted wires with a thin insulating coating and have successfully developed a method capable of forming an insulating coating on twisted metal conductors without causing blistering or leaving air bubbles entrapped within the coating. As mentioned above, when an insulating coating is formed on twisted conductors by the conventional method of applying and curing insulating paint, air in gaps (indicated at 2 in Fig. 1) between conductors expands upon heating and causes frequent blistering. When air is heated at a given pressure from, for example, room temperature (20°C) to 250°C, its volume increases by a factor of about 1.8. The present invention has been accomplished as a result of the intensive studies conducted by the inventors to develop techniques for preventing the occurrence of a blistered insulating coating resulting from such an increase in air volume to thus eliminate entrapped air bubbles. In accordance with the present invention the process for producing an insulated twisted electric wire comprising a plurality of metal conductors which are twisted together comprises the steps of providing said twisted electric wire with an insulating coating having a thickness in a range of 3 to 100% of the radius of a smallest circle circumscribing said conductors, said insulating coating being formed by two or more cycles of coating and curing of an insulating paint, characterized in that before providing said insulating coating, applying a paint that hardens upon radiating by ultraviolet rays or electron beams on said twisted electric wire, hardening the applied paint by said radiation by ultraviolet rays or electron beams, and then applying and curing said insulating paint in cycles to form a plurality of insulating coating layers. If the coating's thickness is less than 3% of the radius of the circumscribing circle, a highly reliable insulated wire will not be obtained, and if the thickness of the insulating coating is more than 100% of the radius of the circle, the method of the present invention provides no specific benefit as conventional extrusion techniques will serve as well. In accordance with the process of the present invention, a radiation-settable paint is applied to the twisted conductors, and the applied layer is then hardened so as to produce an insulated twisted wire having no blistered insulating coating. A further embodiment of the present invention includes an invention directed to the elimination of air bubbles from the applied insulating layer before it hardens. Fig. 1 is a cross section of a set of twisted conductors having a coating of insulating paint; Fig. 2 is a cross section of an insulated twisted wire fabricated in accordance with the method of the present invention, wherein twisted conductors are first coated with a solvent-free radiation-settable paint, and after curing the applied layer, the conductors are coated with an insulating paint, followed by curing the applied coating; Fig. 3 is a cross section of an insulated twisted wire which has air bubbles contained in the insulating coating since twisted conductors have been immediately introduced into a bath of solvent-free radiation-settable paint without first passing them through a vacuum compartment; Fig. 4 is a temperature vs. viscosity curve for the solvent-free radiation-settable paint; and Fig. 5 is a side elevational view of a paint bath having a vacuum compartment positioned below so that twisted conductors will first pass through that vacuum compartment before they are introduced into the paint bath. The above-mentioned aspects of the present invention are hereunder described in more detail. When twisted conductors are provided with an insulating coating by drawing them through a die subsequent to a tank of insulating paint, the die of course must have a bore diameter larger than the outside diameter of the smallest circle that circumscribes the set of twisted conductors. This provides an unnecessarily thick coating between conductors, and the expansion of air in the gaps between conductors (see Fig. 1 reference no. 2) and the evaporation of the solvent in the paint will lead to an increased chance of blistering of the finally obtained insulating coating. As a matter of fact, twisted conductors having a blistered, and hence hard, coating will often break to made further wire fabrication impossible. A thinner insulating coating could be formed by using a die having a bore size substantially equal to the outside diameter of the smallest circle that circumscribes the set of twisted conductors, but this causes rapid wear of the die, or variations in the outside diameter of the circumscribing circle along the length of the conductors introduce an unevenness in the friction between conductors and the die, making it impossible to provide a uniform insulating coating over the entire length of the conductors. In accordance with the process of the present invention, a solvent-free radiation-curable paint which will harden at room temperature is applied in layers to twisted conductors until a suitable film thickness is obtained, and the applied layers are thereafter cured by radiating with ultraviolet rays or electron beams so as to entrap air within the space defined by the set of twisted conductors. Subsequently, in order to provide an insulated twisted wire having the desired overall thickness of insulating coating and which exhibits the desired wire characteristics, an insulating paint, which may be either the same as or different from the radiation-hardenable paint, is applied and cured. In accordance with the present invention, no blistering of the insulating coating results from the expansion of air in the gaps between conductors, and an insulated twisted wire that can satisfactorily withstand actual service is obtained. The film of the solvent-free radiation-curable paint should cover the hatched area 8 in Fig. 2, and it may be applied with a felt applicator, or by passing through a die, or by any other known technique. The film at 8 in Fig. 2 need not be formed by a single application, and curing of the radiation-curable paint and multiple coating and curing cycles may be repeated in order to provide that film. As will be apparent from the foregoing description, the problem associated with blistering of an insulating coating that results from the expansion of air in the gaps between the twisted conductors is eliminated by employing the method described above. The present invention also provides in preferred embodiments two methods for eliminating air bubbles from the applied insulating paint before it hardens. In both methods, at least one layer of a paint that hardens upon radiating by UV rays or electron beams is applied and subsequently cured. In accordance with one of these methods, the conductors provided with a layer of such paint are passed through a heating chamber in order to eliminate any tiny air bubbles that would otherwise remain in the unhardened layer, which is then cured by radiating by UV rays or electron beams. If the radiation-curable paint is applied to the twisted conductors with a felt applicator or by passing through a die, tiny air bubbles present in the paint will not be eliminated by the squeezing action between the inner surface of the die and the conductors, and will be left in the applied layer. If the layer is immediately radiated by UV rays or electron beans, tiny air bubbles will remain in the hardened layer. The inclusion of such tiny bubbles will cause variations in the electrical properties of the final wire and should be eliminated. Fig. 3 shows a cross section of an insulated twisted wire having air bubbles entrapped within the insulating coating. As twisted conductors are continuously fed through a bath of radiation-curable paint, air bubbles gradually build up in the paint. The source of these air bubbles is the air that is carried on the conductors and entrapped in the paint. With ordinary bakable insulating paints, heating in the baking chamber causes air bubbles to expand in the applied layer and reduces the viscosity of the paint, thus permitting the expanded air bubbles to pass through the paint layer and disappear. If the insulating layer is further heated, air in the gaps between conductors will expand and pass to the outside and escape through the layer. As a result, the final insulating coating may have blisters in its surface but no tiny air bubbles left in its interior. On the other hand, the radiation-curable paint which is cured as soon as it is applied allows insufficient time for the air bubbles to reach the surface of the applied layer. In order to solve these problems, the present invention provides that after applying a radiation-curable paint to twisted conductors, intentionally heated them so that the viscosity of the paint would be sufficiently reduced to allow for the air bubbles to float to the surface of the applied layer. Subsequently, the layer was hardened by radiating by either UV rays or electron beams. As was initially expected, the resulting insulating coating had no air bubbles. The heating conditions necessary for reducing the viscosity of the paint and allowing for the air bubbles to float to the surface of the applied coating should be properly determined in accordance with the viscosity vs. temperature characteristics of the paint, the coating thickness, and the wire drawing speed. Generally, a furnace of a length of 1 to 2 m which is held at a temperature in the range of 100 to 250°C will serve the purpose. In this case, the twisted wire is preferably held at a temperature between 60 and 150°C. If the furnace temperature is too high, air in the gaps between the twisted conductors may be expanded thermally and will remain in the final coating as air bubbles. If the radiation-curable paint used has such viscosity vs. temperature characteristic that its viscosity adequately decreases with increasing temperature as shown in Fig. 4, the surface of the paint coating layer will soon become smooth after the air bubbles have been released therefrom. However, a coating with an uneven surface may result from a paint whose viscosity is not reduced by a sufficient degree with increasing temperature to provide a smooth-surfaced coat. If such a paint is to be used, some provision must be made to provide a smooth-surfaced coating, for example, by slowing the curing rate. The number of layers of radiation-curable paint to be applied should vary with the desired coating thickness. If the thickness of a coating later formed by a single application is in the range of about 10 to 20 microns, any air bubbles in the paint will disappear as a result of the drop in the viscosity of the paint, following the subsequent heating. In accordance with the other method for eliminating air bubbles from the applied coating of heat-curable paint, a vacuum compartment is provided below the bath of paint and the twisted conductors are passed through this vacuum compartment so as to remove any air from the area surrounding the conductors. By subsequently introducing the conductors into the paint bath, a coating layer of the radiation-curable paint that is entirely free from air bubbles is formed on the twisted conductors. One advantage of this method is that there is no need to provide a heating chamber subsequent to the coating step. The purpose of the vacuum compartment provided below the paint bath is to eliminate air that has been introduced by the twisted conductors and to introduce the air-free conductors into the paint bath. The amount of air bubbles that will enter the paint bath is reduced as the pressure in the vacuum compartment becomes lower than one atmosphere. Preferably, the pressure in the vacuum compartment is lower than 2.10⁴ Pa (150 mmHg), and at such low pressures, the inclusion of air bubbles in the insulating coating is eliminated almost completely and an insulated twisted wire having stable electrical characteristics can be obtained. If the pressure in the vacuum compartment is higher than 2.10⁴ Pa (150 mmHg), very small air bubbles may be incorporated in the final insulating coating. It should, however, be noted that as the pressure in the vacuum compartment is reduced, it becomes more likely that the paint in the bath will be drawn into the vacuum compartment. In order to avoid this backflow of paint, an elastic packing such as one made of rubber is provided at the interface between the paint bath and the vacuum compartment, and at the same time, the aperture in the bottom wall of the paint bath through which the twisted conductors are to pass should have a sufficiently small diameter as to avoid the packing from becoming deformed by the suction created under reduced pressure. Additionally, as the pressure in the vacuum compartment is reduced, the chance of air entering the vacuum compartment through the passage hole or aperture in the bottom wall is increased. This must be prevented, for example, by placing an elastomer such as rubber on the bottom of the vacuum compartment. Even if the pressure in the vacuum compartment is reduced, the elasticity of the rubber will decrease the diameter of the aperture through which the twisted conductors passes. If desired, the elastomer can be reinforced with an underlying plate so that it will satisfactorily withstand the suction developed during evacuation of the vacuum compartment. Needless to say, the aperture in the reinforcing plate through which the twisted conductors are to pass should have the smallest diameter. The longer the twisted conductors take to pass through the vacuum compartment, the more air can be eliminated from the surface of the conductors. The present inventors have confirmed by experiment that a vacuum compartment with a length in the range of about 5 mm to 10 cm should suffice. Fig. 3 shows a cross section of an insulated twisted wire fabricated by drawing twisted conductors through a paint bath having no vacuum compartment below. In Fig. 3, reference numeral 1 denotes individual twisted conductors, 2 is a gap between conductors, 8 is an insulating coating formed by applying and curing a solvent-free radiation-curable paint, 9 is an overlying insulating coating, and 10 is an air bubble. As illustrated in Fig. 3, the likelihood that tiny air bubbles are entrapped within the coating of radiation-curable paint is high if no vacuum compartment is provided below the paint bath. Fig. 5 illustrates a paint bath 14 that is equipped below with a vacuum compartment 15. As shown, a set of twisted conductors 11 is first introduced into the vacuum compartment before passing through the paint bath. The interior of the vacuum compartment 15 may be evacuated with a pump 17 capable of reducing the pressure in the compartment to less than 2.10⁴ Pa (150 mmHg). The top and the bottom of the vacuum compartment are each sealed with a packing 18, and the bottom of the compartment is reinforced by an underlying plate 16. By causing the twisted conductors to pass through the vacuum compartment prior to their introduction into the paint bath, the amount of air bubbles that enter the paint bath is drastically reduced, and substantially no air bubbles are included in the cured coat of radiation-curable paint. Examples of insulating paints indicated by reference sign 8 in figs. 2 and 3 that can be used in the practice of the present invention and which are capable of curing upon radiation by UV rays or electron beams include those which are based on polyester acrylate, polyol acrylate, urethane acrylate, epoxy acrylate, silicone acrylate, polybutadiene acrylate, melamine acrylate, polyene/polythiol and unsaturated polyester. These polymers as paint bases may be used either alone or in admixtures. The radiation-curable paints listed above must contain photosensitizers if they are to be hardened by radiation with UV rays. Any of the known photosensitizers may be used, which include benzoin alkyl ethers such as benzoin ethyl ether and benzoin-n-butyl ether, acetophenone derivatives such as diethoxyacetophenone, and amyl oxime esters. The insulating coating indicated by reference sign 9 in Figs. 2 and 3 may be formed from any known insulating paint such as those based on polyvinylformal, polyurethane, polyester, polyester imide, polyamideimide and polyimide; hot-melt type insulating paints; and radiation-curable paints. These paints may be used either independently or in admixtures. The twisted conductors to be provided with a thin insulating coating in accordance with the present invention may be made of any common conducting materials such as copper, copper alloys, tin-plated copper and solder-plated copper. In Figs. 1 to 3, seven conductors are twisted together but this is only an example and a smaller or greater number of conductors may be twisted together. There is also no limitation on the size of the metal twisted conductors that can be treated in accordance with the present invention. The following Examples and Comparative Examples are provided for further illustration of the claimed method and should not be construed as limiting. Comparative Example 1 A set of seven twisted copper conductors (0.06 mm in diameter) was coated with a polyurethane base insulating paint (viscosity: 4,000 cps, concentration: 40%) by passing through a die, and the applied layer was subsequently baked at 300°C. The wire speed was 20 m/min. Such coating and baking cycles were repeated five times. The resulting insulating coating had an average of three to 10 blisters per meter of wire. The characteristics of the insulated wire are shown in Table 1. Comparative Example 2 A set of seven twisted copper conductors (0.05 mm in diameter) was coated with a polyester base insulating paint (viscosity: 3,500 cps, concentration: 40%) by passing through a die, and the applied layer was subsequently baked at 320°C. The wire speed was 20 m/min. Such coating and baking cycles were repeated eight times. The resulting insulating coating had an average of two to seven blisters per meter of wire. The characteristics of the insulated wire are shown in Table 1. Comparative Example 3 A set of seven twisted tin-plated copper conductors (0.127mm in diameter) was coated with a solvent-free radiation-curable paint (viscosity: 3,500 cps at 30°C) by passing through a die. The paint was Aronix 6100 (an ester acrylate oligomer of Toagosei Chemical Co., Ltd., in Japan) and 1.5 wt% of a photosensitizer (Sundray #1000 of Mitsubishi Petrochemical Company, Ltd., in Japan). The applied layer was subsequently hardened by exposing to a 3 kW ultraviolet lamp. The wire speed was 20 m/min. Such coating and curing cycles were repeated four times. The resulting insulating coating contained three to 20 tiny (about 10 microns in diameter) air bubbles per meter of wire. The characteristics of the insulated wire are shown in Table 2 below. Comparative Example 4 A set of seven twisted copper conductors (0.127 mm in diameter) was coated with a solvent-free radiation-curable paint (viscosity: 5,200 cps at 30°C) by passing through a die. The paint was a 1:1 mixture of VR-90 (epoxy acrylate oligomer of Showa Highpolymer Co., Ltd., in Japan) and Aronix 6100 (ester acrylate oligomer of Toagosei Chemical Co., Ltd.). The applied layer was hardened by exposing to a total dose of 7 Mrad of electron beams in a nitrogen atmosphere. The wire speed was 20 m/min. Such coating and curing cycles were repeated four times. The resulting insulating coating contained 10 to 30 tiny (about 10 microns in diameter) air bubbles per meter of wire. The characteristics of the insulated wire are shown in Table 2. Comparative Example 5 The procedures of Comparative Example 4 were repeated except that the twisted conductors were introduced into the paint bath after passing through a vacuum compartment held at 300 mmHg. The resulting insulating coating contained five to 20 tiny (about 10 microns in diameter) air bubbles per meter of wire. The characteristics of the insulated wire are shown in Table 2. Example 1 The procedures of Comparative Example 1 were repeated except that the twisted conductors were first coated with a solvent-free radiation-curable paint (for its composition, see Comparative Example 5) by means of a felt applicator, followed by curing of the applied layer by exposing to a 3 kW UV lamp. The wire speed was 20 m/min, and the coating and curing cycles were repeated twice. Thereafter, the conductors were coated with a polyurethane base insulating paint as shown in Comparative Example 1. The resulting insulated twisted wire had an insulating coating having a good appearance with no blisters. The characteristics of the wire are shown in Table 1. Example 2 The procedures of Comparative Example 2 were repeated except that the twisted conductors were first coated with a solvent-free radiation-curable paint (for its composition, see Comparative Example 6) by means of a felt applicator, followed by curing of the applied layer by exposing to a total dose of 7 Mrad of electron beams in a nitrogen atmosphere. The wire drawing speed was 20 m/min, and only one coating and curing cycle was performed. Thereafter, the conductors were coated with a polyester base insulating paint as shown in Comparative Example 2. The resulting insulated twisted wire had an insulating coating having a good appearance with no blisters. The characteristics of the wire are shown in Table 1. Example 3 The procedures of Comparative Example 3 were repeated except that the conductors coated with a solvent-free radiation-curable paint were passed through a heating chamber (230°C, 1.5 m long) before the coating was cured by exposing to UV rays. The resulting insulating coating contained no small air bubbles. The characteristics of the insulated twisted wire are shown in Table 2. Example 4 The procedures of Comparative Example 4 were repeated except that the twisted conductors coated with the solvent-free radiation-curable paint were passed through a heating chamber (240°C, 1.5 m in length) before the applied layer was hardened by exposing to electron beams in a nitrogen atmosphere. The resulting insulating coating did not contain any small air bubbles. The characteristics of the insulated twisted wire are shown in Table 2. Example 5 The procedures of Comparative Example 3 were repeated except that the twisted conductors were passed through a vacuum compartment (80 mmHg) before they were introduced into the paint bath. Since no air bubbles entered the paint bath, a cured insulating coating having no air bubbles was obtained. The characteristics of the insulated twisted wire are shown in Table 2. Example 6 The procedures of Comparative Example 4 were repeated except that the twisted conductors were passed through a vacuum compartment (100 mmHg) before they were introduced into the paint bath. Since no air bubbles entered the paint bath, a cured insulating coating having no air bubbles was obtained. The characteristics of the insulated twisted wire are shown in Table 2. In Comparative Examples 1 to 2, the twisted conductors were coated with highly viscous insulating paints by passing through a die, and the resulting insulating coatings had many blisters. In Example 1, the polyurethane base insulating paint was applied to the twisted conductors after the paint that was curable upon radiating by UV rays was applied and cured. In Example 2, the polyester base insulating paint was applied to the twisted conductors after the paint that was curable upon radiating by electron beams had been applied and cured. No blistering occurred in either of the insulating coatings formed in Examples 1 and 2. In Comparative Examples 3 and 4, wherein paints curable by exposing to UV rays or electron beams were respectively applied to the twisted conductors and subsequently cured, the insulating coatings obtained had no blisters but contained many air bubbles. In Examples 3 and 4, the twisted conductors having coatings of radiation-curable paints were passed through the heating chamber before the coatings were cured. Since any air bubbles present in the coatings were eliminated during the passage through the heating chamber, the finally obtained insulating coating contained no air bubbles. In Comparative Example 5, the conductors were passed through a vacuum compartment before they were introduced into the paint bath, but the pressure in that compartment was 300 mmHg, that is, a pressure higher than 150 mmHg, the preferred value for the purposes of the present invention. Therefore, the cured insulating coating contained a significant number of air bubbles, although they were not as many as in the coating of Comparative Example 4. In Examples 5 and 6, the pressures in the vacuum compartment were respectively 30 mmHg and 50 mmHg, well below the preferred value of 2.10⁴ Pa (150 mmHg). Therefore, the insulating coatings prepared in these Examples were entirely free from air bubbles.

Claims: A process for producing an insulated twisted electric wire comprising a plurality of metal conductor (1) which are twisted together, said process comprising: providing said twisted electric wire with an insulating coating (9) having a thickness in a range of 3 to 100% of the radius of a smallest circle circumscribing said conductors, said insulating coating being formed by two or more cycles of coating and curing of an insulating paint, characterised in that before providing said insulating coating (9), applying a paint (8) which hardens upon radiating by ultraviolet rays or electron beams on said twisted electric wire, hardening the applied paint by said radiation of ultraviolet rays or electron beams, and then applying said insulating coating (9) by applying and curing said insulating paint in cycles to form a plurality of insulating coating layers (9). The process according to Claim 1, further comprising the step of passing said twisted electric wire to which said radiation-hardenable paint has been applied through a heating chamber so as to eliminate gas bubbles in said applied paint, and thereafter, exposing said electric wire to ultraviolet rays or electron beams to harden said paint. The process according to claim 1, further comprising the step of passing said twisted electric wire through a vacuum compartment to eliminate air from an area surrounding said electric wire, and then immediately introducing said electric wire into a bath of radiation-hardenable paint to apply said paint, and subsequently hardening said radiational-hardenable paint by exposure to ultraviolet rays or electron beams. The process according to Claim 3, wherein a pressure in said vacuum compartment is lower than 2.10⁴ Pa (150 mmHg).

Inventors: KATSUDA, MORIHIKO C/O NAGOYA WORKS, MASUDA, SHIGEO C/O NAGOYA WORKS, UEOKA, ISAO C/O NAGOYA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1985
Main classification: H01B13
IPC codes: H01B7, H01B13