Method of Power Inserting Polyamide-Imide Coated Magnet Wire

The field of art to which this invention pertains is lubricant coatings for electrical conductors, and specifically lubricant coated magnet wire.

In the manufacture of electrical motors, the more magnet wire which can be inserted into a stator core, the more efficient the motor performance. In addition to motor efficiency considerations, motor manufacturers are also interested in manufacture efficiency. Accordingly, such coils where possible are inserted automatically, generally by two methods: either a gun-winding method or a slot insertion method. In the older gun-winding method, the winding is done by carrying the wire into the stator slot by means of a hollow winding needle. Turns are made by the circular path of the gun to accommodate the individual coil slots. As described in Cal Towne's paper entitled "Motor Winding Insertion" presented at the Electrical/Electronics Insulation Conference, Boston, Mass. in September, 1979, in the more preferred slot insertion method, coils are first wound on a form, placed on a transfer tool and then pressed off the transfer tool into the stator core slots through insertion guides or blades. In order to accommodate these automated insertion methods, wire manufacturers have responded by producing magnet wires with insulating coatings with low coefficients of friction. Note, for example, U.S. Pat. Nos. 3,413,148; 3,446,660; 3,632,440; 3,775,175; 3,856,566; 4,002,797; 4,216,263; and Published European Patent Application No. 0-033-244, published Aug. 5, 1981 (Bulletin 8/31).

With the availability of such low friction insulating coatings motor manufacturers began to take advantage of such coatings by inserting an increasing number of wires per slot into the motors. However, it was also well known in this art that there existed a locking wire size range where based on the size of the insulated wires themselves, attempts at inserting a certain number of wires into a particular size slot opening at one time caused a wedging action of the wires with resulting damage to the coated wires. In spite of this fact, in the interest of efficiency and a better product, motor manufacturers continue to insert in a range closely approaching the locking wire size range even though discouraged from doing so by power insertion equipment manufacturers. And while nylon overcoated wires have been known to be successfully inserted in a locking wire size range, polyamide-imide overcoated wires, although making superior magnet wire products (e.g. in water resistance and temperature stability) have not been successfully power inserted in the locking wire size range.

Accordingly, what is needed in this art, is an insulated magnet wire having a polyamide-imide insulation coating which can be power inserted into a coil slot in the locking wire size range without damage to the wire.

The present invention is directed to magnet wire having an outermost insulating layer of polyamide-imide overcoated with an external lubricant coating which allows it to be reliably power inserted into a coil slot in its locking wire size range without damage to the insulation. The lubricant comprises a mixture of paraffin wax, hydrogenated triglyceride, and esters of fatty acids and fatty alcohols.

Another aspect of the invention is directed to wire as described above additionally containing in the polyamide-imide insulation layer an internal lubricant comprising esters of fatty acids and fatty alcohols.

Another aspect of the invention includes the method of producing such lubricated wires by applying the external lubricant composition in solution to the polyamide-imide insulation and drying the coated wire.

Another aspect of the invention includes the method of power inserting such wires into coil slots.

The foregoing, and other features and advantages of the present invention, will become more apparent from the following description and accompanying drawing.

The FIGURE demonstrates power insertion locking wire size range as a function of coil slot opening size.

It is important to use the components of the lubricant composition according to the present invention in particular proportions. In solutions in aliphatic hydrocarbon solvent, the paraffin wax should be present in an amount about 0.1% to about 4% by weight, the hydrogenated triglyceride present in about 0.1% to about 10% by weight, and the esters of fatty alcohols and fatty acids present in about 0.1% to about 10%, with the balance being solvent. The preferred composition comprises by weight about 1% paraffin wax, about 0.5% hydrogenated triglyceride, and about 0.5% esters of fatty alcohols and fatty acids, with balance solvent. While solution application is preferred, if solventless (i.e. molten) application is used, the paraffin and triglyceride should be used in a ratio by weight of about 1:15 to 60:1 and preferably of about 2:1 and the triglycerides and esters in a ratio of about 1:30 to about 30:1 and preferably of about 1:1. The paraffin wax is preferably petroleum based having a melting point of 122.degree. F. to 127.degree. F. (50.degree. C. to 52.8.degree. C.). Eskar R-25 produced by Amoco Oil Company, having a refractive index of 1.4270 at 80.degree. C., an oil content of 0.24%, specific gravity (at 60.degree. F., 15.6.degree. C.) of 0.839 and a flash point of 415.degree. F. (212.8.degree. C.) has been found to be particularly suitable.

The hydrogenated triglyceride is aliphatic hydrocarbon solvent soluble and has a melting point of 47.degree. C. to 50.degree. C. A hydrogenated triglyceride which has been found to be particularly suitable is Synwax #3 produced by Werner G. Smith, Inc. (Cleveland, Ohio) having an Iodine No. of 22-35, a Saponification No. of 188-195, an Acid No. of 5 (maximum) and has approximate fatty acid component proportions of C.sub.14 fatty acids--8%, C.sub.16 fatty acids--34%, C.sub.18 fatty acids--27%, C.sub.20 fatty acids--16%, and C.sub.22 fatty acids--15%. A fatty acid ester composition which has been found to be particularly suitable is Smithol 76 produced by Werner G. Smith, Inc., which has a Saponification No. of 130-140, an Iodine No. of 85-95 and comprises (in approximate proportions) C.sub.12 to C.sub.14 fatty alcohol esters of tall oil fatty acids (54.6%), tri-pentaerythritol esters of tall oil fatty acids (24.5%), tetra-pentaerythritol esters of tall oil fatty acids (9.8%), free tall oil fatty acids (6.3%) and free C.sub.12 to C.sub.14 alcohols (4.8%).

The solvents for the solution applications of the lubricant composition according to the present invention are preferably aliphatic hydrocarbons with a rapid vaporization rate, but a flash point which is not so low as to present inordinate flammability dangers. Aliphatic hydrocarbons such as naphtha, heptane and hexane can be used. Lacolene.TM. produced by Ashland Chemical Company, an aliphatic hydrocarbon having a flash pont (Tag closed up) of 22.degree. F. (-5.6.degree. C.), an initial boiling point of 195.degree. F. (90.6.degree. C.) a boiling range of 195.degree. F. (90.6.degree. C.) to 230.degree. F. (110.degree. C.), a specific gravity at 60.degree. F. (15.6.degree. C.) of 0.6919 to 0.7129, and a refractive index at 25.degree. C. of 1.3940 has been found to be particularly suitable. To reduce flammability dangers, any of the above materials may be used in admixture with Freon.RTM. solvents (duPont de Nemours and Co., Inc.).

Preferably, a small amount of esters of fatty alcohols and fatty acids which are unreactive and insoluble in the cured polyamide-imide can be added to the polyamide-imide insulation layer to further improve power insertability of the treated wires. Because of the insolubility of the fatty acid ester composition in the cured polyamide-imide film, it will exude to the surface of the film, further enhancing power insertion in the locking wire size range. The fatty acid ester composition is added to the polyamide-imides in amounts of about 0.05% to about 8% by weight, with about 1% being preferred. The fatty acid ester composition can be added to the amide-imide enamel composition either as it is being formulated or after formulation and prior to application to the wire. In the latter case, the enamel composition should be heated up slightly above room temperature to aid in uniform mixing of the ester composition in the enamel.

As the electrical conducting base material, any electrical conductor which requires a lubricant can be treated according to the present invention, although the invention is particularly adapted to wire and specifically magnet wire. The wire is generally copper or aluminum ranging anywhere from 2 to 128 mils in diameter, with wires 10 mils to 64 mils being the most commonly treated wires according the present invention. The insulating wire coatings to which the lubricant is applied generally ranges from about 0.2 to about 2 mils in thickness, and generally about 0.7 mil to 1.6 mils. The polyamide-imide is that conventionally used in this art and can be applied as a sole insulation coat or part of a multicoat system. Although any compatible base coat material can be used as part of the multicoat system, tris-hydroxyethyl-isocyanurate based polyester (preferably representing about 80% to about 90% by weight of the total wire coating) is the preferred base coat in conjunction with the polyamide-imide (preferably representing about 10% to about 20% by weight of the total wire coating) overcoat.

The external lubricant can be applied by any conventional means such as coating dies, rollers or felt applicators. The preferred method of application utilizes a low boiling hydrocarbon solvent solution of the lubricant which can be applied with felt applicators and air dried, allowing a very thin "wash coat" film of lubricant to be applied to the wire. While the amount of lubricant in the coating composition may vary, it is most preferred to use approximately 1% to 3% of the lubricant dissolved in the aliphatic hydrocarbon solvent. And while any amount of lubricant coating desired can be applied, the coating is preferably applied to represent about 0.003% to about 0.004% by weight based on total weight of wire for copper wire, and about 0.009% to about 0.012% for aluminum wire.

A copper wire approximately 22.6 mils in diameter was coated with a first insulating layer of a THEIC based polyester condensation polymer of ethylene glycol, tris-hydroxyethyl isocyanurate and dimethylterephthalate. Over this was applied a layer of a polyamide-imide condensation polymer of trimellitic anhydride and methylene diisocyanate. The insulating layers were approximately 1.6 mils thick with 80% to 90% of the coating weight constituted by the polyester basecoat, and 10% to 20% by the polyamide-imide topcoat.

500 grams of paraffin wax (Eskar R-25) and 250 grams of hyrogenated triglyceride (Synwax #3) and 250 grams of esters of fatty acids and fatty alcohols (Smithol 76) were added to approximately 9844 grams of aliphatic hydrocarbon solvent (Lacolene). The resulting solution had a clear appearance, a specific gravity at 25.degree. C. of 0.7131.sup..+-. 0.005, and an index refraction at 25.degree. C. of 1.4034. The solvent was heated above room temperature, preferably to a point just below its boiling point. The paraffin wax was slowly brought to its melting point and added to the warm solvent. The hydrogenated triglyceride and fatty acid ester composition were similarly slowly brought to their melting point and added to the warm solvent. The blend was mixed thoroughly for 5 minutes. The polyamide-imide overcoated THEIC polester wire was run between two felt pads partially immersed in the above formulated lubricant composition at a rate of about 70 feet to 80 feet per minute (21 M/min to 24 M/min) and the thus applied coating air dried. The lubricant represented about 0.003% to about 0.004%, by weight of the entire weight of the wire.

The same procedure followed in Example 1 was performed here, with the exception that 1% by weight based on total weight of the polyamide-imide insulating layer was comprised by esters of fatty acids and fatty alcohols (Smithol 76). The fatty acid ester composition was added to the amide-imide enamel when it was in solution prior to the application to the wire. Multiple windings of the thus lubricated wire were power inserted simultaneously into the stators in its locking wire size range with no damage to the insulated magnet wire. As can be clearly seen from the FIGURE, where the area A on the curve represents the locking wire size range as a function of insertion bladed coil slot opening (coil slot opening less 0.8 mm), for this wire size and coil slot size the coated wire was clearly within locking wire size range and yet inserted with no problem. In effect, what the lubricated wires according to the present invention have accomplished is to shrink area A in the FIGURE to the point of eliminating locking wire size restrictions for power insertable magnet wires according to the present invention.

As described above, problems have been incurred with the use of lubricant coated magnet wire in attempts to power insert in the locking wire size range. Previously, it was felt that conventional coefficient of friction testing was sufficient for predicting the feasibility of power inserting a particular magnet wire into coil slots. However, it has now been found that perpendicularly oriented wire to wire, and wire to (insertion blade composition and polish) metal, coefficient of friction data at increasing pressure levels are necessary for true power insertion predictability. For example, in conventional coefficient of friction tests where both lubricant treated nylon and lubricant treated polyamide-imide coatings had identical coefficients of friction measurements, the nylon could be made to successfully power insert and the polyamide-imide couldn't. The compositions of the present invention provide the necessary increasing pressure coefficient of friction properties to the insulated magnet wires for successful power insertion predictability.

While many of these components have been used as lubricants, and even as lubricants in the insulated electrical wire field, there is no way to predict from past performance how such lubricants would react to power insertion in coil slots in the locking wire size range specifically cautioned against by power insertion equipment manufacturers. Accordingly, it is quite surprising that the combination of such conventional materials in the ranges prescribed would allow power insertion of polyamide-imide material hitherto believed to be incapable of successful power insertion in the locking wire size range.

Magnet wire in this environment must also be able to maintain a maximum voltage level even in high humidity or "water test" conditions. Since polyamide-imide insulated magnet wires are known to be more water resistant than nylons, the lubricant of the present invention provides this additional benefit in the area of power insertable wire. Another important advantage with lubricants according to the present invention is in the area of hermetic motors. In the past, the use of lubricant coated, power inserted coils has been avoided in this area because of the potential for clogging of capillary tubes by the lubricant in the environment the hermetic motors are used in. However, the lubricants according to the present invention are substantially 100% removed in the course of the ordinary 300.degree. F. (150.degree. C.), eight hour varnish curing operation in the hermetic motor manufacturing process.

Although the invention has been primarily described in terms of the advantage of being able to power insert magnet wire according to the present invention in its locking wire size range, the lubricants of the present invention impart advantages to the magnet wires even when they are inserted outside the locking wire size range, and even when the magnet wires are not intended to be power inserted at all. For those magnet wires which are power inserted outside the locking wire size range, less damage is imparted to the wires as compared to similar wires with other lubricants, and it is possible to insert at lower pressures which further lessens damage to the wires. This results in a much lower failure rate (e.g. under conventional surge failure testing) for power inserted coils made with wire according to the present invention than with other lubricated wires. And for those wires which are not power inserted, much improved windability is imparted to such wires, also resulting in less damage to the wires than with other lubricants.

Furthermore, although only particular compositions are specifically disclosed herein, it is believed that as a class, esters non-reactive with and insoluble in the cured polyamide-imide insulation, resulting from reaction of C.sub.8 to C.sub.24 alcohols having 1 to 12 hydroxyls with C.sub.8 to C.sub.24 fatty acids including some portions containing free alcohol and free acid can be used as lubricants according to the present invention, either admixed with paraffin as an external lubricant, or alone (or as admixtures themselves) as internal lubricants. These materials can also be hydrogenated to reduce their unsaturation to a low degree. It is also believed from preliminary testing that C.sub.12 to C.sub.18 alcohols and mixtures thereof are similarly suitable lubricants for use according to the present invention. However, even in this broad class only particular combinations have been found acceptable. Although not desiring to be limited to any particular theory it is believed that factors responsible for this are (1) the potential of the lubricants to interact in molecular fashion with the metal contact surface, e.g. the metal of the insertion blades, and (2) the ability of the lubricant to be or become liquid and stable under pressure condition, e.g. in the insertion process.

Although the invention has been shown and described with respect to detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and scope of the claimed invention.