Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
1999-11-23
2002-10-15
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S379000, C174S1100FC, C174S1200SR
Reexamination Certificate
active
06465097
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an insulated wire used for a coil to be molded with resin after being coiled, for instance.
2. Description of the Background Art
Electrical equipment of automobiles, electronic devices, and other devices that use insulated wires have been required in recent years to be smaller, lighter, and higher in performance in order to enhance energy saving and space saving. To meet the requirements, coils used for the above-mentioned equipment and devices have become high in density of the winding of insulated wires and high in operating current density. Because these devices have been used at higher ambient temperatures, coiled insulated wires have been used inevitably at higher ambient temperatures. Because high winding density has been achieved by squeezing the insulated wire into the enclosure of electronic or other devices, the possibility of damaging the insulating coating of the wire during the winding process has increased. Under such circumstances, the following problems have occurred in flyback transformers used for the CRT of TVs and personal computers and in resin-molded coils such as ignition coils for automobiles:
(a) Damage in insulating coating causes failure in insulation between the conductors or between the conductor and the ground, resulting in unsatisfactory electrical characteristics of the device, and
(b) Higher operating ambient temperature decreases the lifetime of the coil. A study on the cause of the lifetime reduction in resin-molded coils revealed that coils are subjected to heat cycles between room temperature and operating ambient temperature, and their lifetime is reduced by the action of strain caused by the heat cycles.
Lead wires for motor coils whose conductor is covered with a rubber/plastic insulating material have had another problem; when the coils are impregnated with varnish, if the varnish adheres to a lead wire, the lead wire becomes stiff so that bending of the wire produces cracks not only in the varnish but also in the insulating layer at the same time. Techniques have been developed to prevent this problem by forming a coating on the insulating layer with a material having good releasing quality such as fluororesin, silicone, lubricant, and polyethylene glycol (see Published Japanese Utility Model Application Jitsukaishou 61-156114, for example).
The present inventors extracted an idea from these techniques and studied lifetime prolongation of resin-molded coils by forming releasing layers made of various materials on the surface of insulated wires. However, some were insufficient in releasing quality and some were mechanically weak as a releasing layer so that parts of the releasing layer fell off in the winding process, producing unstable quality of adhesion with the molded resin in the longitudinal direction of the insulated wire. No releasing layer tested succeeded in prolonging the lifetime of the coil as a result.
However, we found during the process of the study that (a) when releasing layers formed on the insulated wires were uneven or when silicone or lubricant was used as a part of the layer, prolongation of the lifetime of resin-molded coils was difficult to achieve and that (b) fluororesin is a comparatively suitable material for releasing purposes.
According to the above-cited invention, various kinds of releasing agents may be used as described above, and a releasing layer usually has a thickness in the range as wide as 0.00001 mm or more, desirably from 0.0001 to 0.1 mm. The reason why releasing agents and the thickness of releasing layers have such a high degree of freedom is attributed to the fact that instances of concurrent cracking of the varnish and insulating coating are relatively limited. More specifically, because thin pieces of impregnating varnish adhere at limited portions to the comparatively thick rubber/plastic coating of insulated wires, the concurrent cracking occurs only when the insulated wire is subjected to a large magnitude of distortion or to abrupt deformation.
With resin-molded coils, on the other hand, because the molded resin is thicker than the insulating coating and covers the entire surface of the coil, even a small amount of distortion resulting from heat cycles or another cause may increase the stress transferred from the molded resin to the insulating coating, producing defective conditions.
SUMMARY OF THE INVENTION
An object of the present invention is to offer a new insulated wire that prevents the reduction in lifetime of resin-molded coils under severe operating conditions such as heat cycles with large temperature differences. To achieve this object, the present inventors have earnestly studied the constitution of insulated wires. Finally, the inventors found that the means described below can prolong the lifetime of coils even when coils formed under severe coiling conditions are subjected to heat cycles with large temperature differences, thereby completing the present invention.
The present invention offers an insulated wire that has, on the insulating coating mainly consisting of thermosetting resin, (a) a coating comprising fluororesin without containing a binder or (b) a coating comprising fluororesin and a binder in which the binder constitutes 20 wt. % or less of the total amount of the fluororesin and binder.
The constitution of the present invention was found through the process described below.
(1) First, a study was made on the cause of the lifetime reduction of coils. Coils are covered entirely with molded resin, and the coils' insulated wire is strongly bonded to the molded resin. When the entire molded resin is subjected to heat cycles, the molded resin and coils expand and contract repeatedly. The molded resin is filled with inorganic fillers such as silica for the molded resin to have a thermal expansion coefficient as close as possible to that of the conductor, copper, for instance, of the insulated wire. However, the thermal expansion coefficient of the molded resin is not the same as that of the insulated wire. Therefore, when the temperature difference in heat cycles increases, a difference appears in the magnitude of expansion and contraction between the two components. This discrepancy produces strains between the molded resin and the insulated wire of the coils. When the heat cycles are repeated, the strains accumulate and produce cracks in the relatively weak coating of the insulated wire. The cracks finally lead to an interlayer shortcircuit of the insulated wire of the coils.
(2) In view of the above failure mechanism, the present inventors first considered that the reduction in the bonding strength between the molded resin and the insulated wire of coils is effective to prevent the reduction in the lifetime of coils. In seeking a method for reducing the bonding strength, the concept of providing a releasing layer on the insulated wire transpired. A fluororesin paint was examined as a material constituting the releasing layer. Fluororesin included in fluororesin paint is less adhesive than other ordinary resins and rarely adheres to other substances. It was expected that when fluororesin was added to the insulating coating of insulated wires or when a coating of fluororesin paint was formed on the insulating coating, coils produced with these wires would have reduced bonding strength between the molded resin and coils. Contrary to the expectation, however, the results of the experiments show that the addition of fluororesin into the coating of insulated wires and the application and baking of an ordinary fluororesin paint failed to sufficiently reduce the bonding strength between the molded resin and insulated wire. In other words, the bonding-strength reduction was insufficient to prevent the lifetime reduction of coils.
(3) After various studies, the present inventors found that a binder usually included in a fluororesin paint impedes the reduction of the bonding strength between the molded resin and insulated wire. It is considered that when a fluororesin paint is ap
Hashimoto Hideyuki
Kurata Masaharu
Ueoka Isao
Gray J. M.
Kelly Cynthia H.
Smith , Gambrell & Russell, LLP
Sumitomo Electric Industries Ltd.
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