Title:
Flexible electrical insulating layer
United States Patent 2386634


Abstract:
The present invention relates to electrical insulating layers and to a process of making the same. An important application of the invention is for the heat-resistant insulating coating of electrical conductors and the invention will be described in this connection. One object of the invention...



Inventors:
Preston, Robinson
Application Number:
US32504340A
Publication Date:
10/09/1945
Filing Date:
03/20/1940
Assignee:
SPRAGUE ELECTRIC CO
Primary Class:
Other Classes:
204/476, 204/489, 204/508, 338/262
International Classes:
C23D5/00; C25D13/00; C25D13/02; H01B3/04; H01B3/08
View Patent Images:



Description:

The present invention relates to electrical insulating layers and to a process of making the same. An important application of the invention is for the heat-resistant insulating coating of electrical conductors and the invention will be described in this connection.

One object of the invention is to provide novel insulating layers which withstand high temperatures and exhibit also at such temperatures high insulating resistivity and dielectric strength.

Another object of the invention is to provide an insulating layer which comprises a high proportion of an inorganic insulating refractory material and which layer is inherently flexible, and of great mechanical strength.

A further object of the invention is to provide layers possessing the above desirable characteristics irrespective of whether they are applied as a coating to a base or are used as independent layers.

A still further object is to provide a refractory insulating coating which is well adherent to a base and resistant to mechanical stresses.

Another object of the invention is to provide an insulating coating which is adapted to be fused to a base and which in fusion does not undergo a shrinkage to an extent deleterious to the properties of the coating.

These and further objects of the invention will appear as the specification progresses.

As a result of an extended study of the factors influencing the physical and electrical characteristics of insulating layers, I have found that to obtain a satisfactory insulating layer of which 'a refractory material is the main constituent and which exhibits a high degree of mechanical strength and flexibility, certain requirements relative to the constituents and the method of deposition thereof must be met.

I have found, for example, that to obtain a coating having the above-described desired characteristics, the refractory material must be held together and bonded to the base to which it is applied, by a binder which exhibits, or can be made to exhibit, a high degree of surface adhesion. Not to impair the electrical characteristics of the refractory material, the binder must also possess low electrical losses, high insulating resistivity and high resistance to humidity.

I have furtherfore found that the group of compounds which exhibit elastic and rubber-like properties, for example, rubber latex, neoprene latex, the butadiene rubbers, polymerized isoprene, derivatives of isoprene, and resins of the acrylic ester type, can be made to meet tne above requirements.

The above compounds all essentially consist of high molecular weight organic polymers with elastic properties and have been jointly classified as "Elastomers" by H. L. Fisher (see article in "Industrial and Engineering Chemistry," volume 31, No. 8, pages 941 to 945 incl.).

I have furtherfore found that to enable these elastomers to exhibit the required high adhesion forces, they have to be in a finely divided form, and that to realize their flill advantage as binders, the elastomer and the frit of the inorganic insulating material must be mixed while both are in a finely divided form. This can be achieved, for example, by suspending the binder and the frit in a suitable suspension medium from which the coating is then electrophoretically deposited on the conductor to be coated.

Why the above requirements have to be met and the exact manner in which the novel results are brought about are not fully known to me. I believe however, that by finely dividing the elastomer and the frit and by simultaneously electrophoretically depositing same, the individual elastomer particles form in the deposited coating strong links, which interconnect the adjacent frit particles and bond them to each other and to the conductor. I have found that in accordance with the invention excellent coatings are obtained which comprise 90% or even more by weight of frit and thus 10% or less of the binder.

While the best results to practice my invention can be obtained by using elastomers as binders, I have found that for certain purposes fairly satisfactory results can also be obtained by using the process of the invention with binders formed of substances which are not true elastomers but can be made to exhibit elastomerlike properties. I have found that certain resins through addition of a high proportion of a plasticizer can be made to exhibit such elastomer-like behavior as a binder. This is for instance the case with polystyrene by adding to it more than 25% (by weight) dibutyl phthalate, polyamylnaphthalene, or similar plasticizing agents.

As has been stated, my invention is of general usefulness to provide insulating layers of inorganic materials which maintain high dielectric strength and insulating resistivity at elevated temperatures. Such layers whether used in themselves or applied to a base, show great flexibility and good mechanical strength.

If my invention is used for dielectric layers or insulating coatings applied to a base, the high adhesiveness of such layers or coatings is of further particular importance.

My invention has the further advantage that when fused coatings are desired, fusion of the coating does not cause such shrinkage of the coating as to deleteriously affect its properties. Thus, fused coatings made according to my invention are free of imperfections, weak spots and other shortcomings.

While it has been heretofore proposed to provide insulating layers consisting of a frit and a binder in a small proportion, such prior layers have not been satisfactory due to the inherent shortcomings of the processes used in their manufacture. For example, it has been proposed to form such layers by extruding a mixture consisting of the frit and binder and of a solvent for the binder, the solvent being required to "wet" the surfaces of the frit particles. Layers so formed cannot, however, be made in thicknesses less than about .02". Furthermore, the layers obtained by this process are non-uniform in thickness and exhibit numerous weak spots and imperfections.

Another method of obtaining insulating layers of the above type, consists in applying to the surface to be coated, a slurry consisting of a frit, a binder, and a solvent for the binder, and thereafter evaporating the solvent. While by this method thin layers can be formed, these layers lack mechanical strength and flexibility, and are imperfect because of the non-uniform distribution of the particles due to the settling out which takes place during the evaporation of the solvent.

A third prior method to provide for such layers consists in applying a layer of fibrous refractory material, such as glass wool, impregnated with a suitable binder. Such a method does not produce a coating having a high proportion of the refractory material, because such fibrous refractory materials contain a large proportion (50% and more) of voids. Furthermore, the layers so obtained are brittle and can only be produced in comparatively large thicknesses.

Unlike the above described layers of the prior art, the layers of the invention possess excellent mechanical strength and flexibility not only in thicknesses of the order of .005", but down to thicknesses of .00025 and less, and even such thin, layers can be obtained with comparative ease.

My Invention finds its chief use for layers less than .005" thick and is particularly directed to the obtainment of such layers.

My invention will be further described with reference to the appended drawing in which: Figure 1 is a schematic illustration of a suitable apparatus for carrying out the process of my invention.

Fig. 2 is an enlarged cross-sectional view of a wire element provided with an insulating coating in accordance with the invention; Fig. 3 is a graph giving the relation between the percentage of the frit in the coating, to the ratio of frit to elastomer of the coating solution.

Referring to Fig. 1, a wire 15 is passed for its coating through a coating cell comprising in a container 10, a coating solution II consisting of suspended particles of a suitable frit and of an elastomer in a suitable suspension medium.

T. TI frt is preferably in particles having a size of 1 to 2 microns and may consist of one.or of a mixture of heat-resistant inorganic insulating materials of a vitreous type, such as porcelain enamel, lead borate or other enamels or borate glasses; or of refractory metal oxides, such as the oxides of aluminum, magnesium and titanium; or of other refractory insulating materials, such as talc, silica, or mica.

As binder, an elastomer is used which is also provided in a finely divided form, and which may be, for example, rubber latex, neoprene latex, butadiene rubber, polymerized isoprene, derivatives of isoprene, and/or resins of the acrylic ester type.

As the suspension medium I usually use water and preferably distilled water, although other poorly conducting media in which neither the frit nor the binder is strongly soluble, can be used.

As a rule, I first prepare or obtain a suspension of the elastomer, and then add to it the frit in the desired proportion by weight.

While any of the elastomers above mentioned. are suitable for my invention, I have found that the resins of the acrylic ester type excel in this respect. These resins are also commercially available in aqueous suspensions, for example, under the trade names of "Appretan A" and "Corial Bottom." S To this suspension I add the frit in the form of finely divided particles in an amount later more fully discussed, so that this mixture forms the coating solution 8.

As a rule, the particles of the frit and elastomer carry negative charges, and the conductor 9i is made the anode of the electrophoretic cell. In some cases, however, the particles carry positive charges, in which case the conductor 8 5 has to be made the cathode of the cell and to facilitate the coating, the particles are made more strongly positive by the addition to the solution of suitable polyvalent cations.

The percentage which the insulating frit material constitutes of the resulting coating, is determined by the ratio of the frit and the binder maintained in the coating solution during the electrophoretic deposition.

This relationship I have shown in Fig. 3 for a coating deposited from a coating solution consisting of an aqueous suspension of porcelain enamel frit and of the elastomer "Appretan A." For other frits and elastomers similar relationships exist.

In Fig. 3 the abscissae represent the ratio by 60 weight of the frit and the elastomer in the coating solution, and the ordinates the percentage of frit in the deposited coating. As appears from the graph, a coating comprising more than 80% frit is obtained with a coating solution in which the ratio of frit to binder is greater than 5 to 1.

If this ratio is about 20 to 1, a coating, in which the frit content is increased to about 95% (by weight), is obtained. Even a coating with such a high proportion of frit, possesses excellent me0chanical strength and flexibility, Referring again to Fig. 1, the container 10 can be made to constitute an electrode of the cell, and for this purpose the cell is made of conducting material, for example, of copper. The other electrode of the cell is the wire 15 to be coated.

The wire 15, for example, a copper wire, is unwound from a spool I1 mounted on a rotatable shaft IT, and passes on its way to the coating cell around a mandrel 18 and over a guide pulley I9.

The wire 15 preferably passes through the coating cell as a loop, reversing at the bottom of the cell, and for this purpose there is provided a pulley 31 supported from the bottom of the container by a bracket 30. On its outgoing end the wire 15 passes over a pulley 20 to a drying oven schematically shown as 21.

In the oven the moisture is removed from the coating in various ways, for example, by passing through the oven a heated air blast from a jet 22 connected to a suitable source (not shown).

From the oven 21 the wire passes to and is wound upon a spool 23.

In case such coated wire is to be used for the winding of coils and the coating is to be fused to the wire, such fusing takes place after the coil has been wound, i. e. after the wire has been bent to its final shape. The temperature required to fuse the coating is determined by the fusion point of the specific refractory frit used.

The electric current for the coating is supplied by a suitable source of direct current, indicated as a battery 24, the negative pole of which is connected to the container 10, and the positive pole of which is connected through an ammeter S 25 to a contact brush 29 which contacts with the mandrel 18. Through the mandrel 18 the wire 15 is thus connected to the positive pole of the source 24. A voltmeter 26 is connected across the source 24.

Suitable driving means (not shown) rotate spools 16 and 23 and cause the wire 15 to unwind from spool 16, pass through the coating cell and the oven, and to wind upon spool 23.

Figure 2 shows an enlarged cross section of an electrical conductor provided with a coating of my invention. The coating comprises frit particles 21 intimately bonded to each other and to the wire 15 by an elastomer binder 28.

I will further illustrate my invention by means of the following specific examples: For the coating of a copper wire .05" in diameter, I prepare a coating solution of 200 cc. of "Appretan A"-consisting of an aqueous suspension of 52 grams of acrylic ester resin-to which I add a frit consisting of 220 grams of porcelain enamel and 94 grams of silica.

Through this coating solution the wire is passed at a speed of one foot per second, whereby a length of two feet of the wire is immersed at a time. By applying a coating voltage of 5 volts across the coating cell, a current of about milliamperes per inch of immersed length of wire passes through the cell and causes a coating of the thickness of .002" to deposit on the wire.

The voltage as well as the time of deposition may be varied, whereby in general the thickness of the deposited coating is roughly proportional to the voltage used, and thus the time of deposition can be reduced by the use of a correspondingly higher voltage.

To dry the wire I use an air blast heated to a temperature of 100* C.

The coating so obtained consists of approximately 85% (by weight) frit and 15% of binder.

A wire so coated can be tied into a knot without any damage to the coating.

A coating comprising the frit above described may be fused to the wire by heating it to a temperature of approximately 750* C. This heating drives off most of the binder, however, in view of the small proportion of the binder, this does not cause such shrinkage of the coating as to deleteriously affect its properties.

By using the same type of wire as above and under the same conditions of coating, but using a coating solution of 200 cc. of water and 100 cc of "Appretan A"-containing 26 grams of acrylic ester resin-to which is added 250 grams of porcelain enamel, I obtain a coating which contain. approximately 95% frit and only 5% of binder.

Even with such a high proportion of frit, the coating obtained exhibits a high degree of flexibility and mechanical strength. The so-coated wire can be bent into a helix about a mandrel 0.4" in diameter, or eight times the diameter of the wire, without any damage to the coating. A coating comprising the above frit fuses at approximately 7250 C.

It should be noted that my invention because of the high degree of flexibility, the high mechanical strength and the low shrinkage in fusion of the coating, has for the first time made it possible to provide coils the insulation of which consists of an integrally fused coating of a refractory material. Coils of even the most complicated shapes can be so provided while the wire is flexible, and in the subsequent fusion of the coating an inflexible and highly stable assembly is formed. Furthermore, while the coating of the invention is particularly adapted for fused integral coatings, coatings of the invention exhibit highly desirable electrical properties even if not fused.

Such coatings are heat resistant, moisture-resistant, have low dielectrical losses and high insulating resistance. A coating formed in accordance with the first above example but without being fused, has a breakdown voltage in excess of 1000 volts per mil of thickness and a resistivity of approximately 1000 megohms.

As above stated, the layers of the invention are also applicable as a dielectric for electrical condensers either as independent layers or as an integral coating on the condenser electrodes. ,An excellent dielectric layer can be provided as an integral coating on the surface of a condenser electrode by electrophoretic deposition from a coating solution of 300 cc. of water, 200 cc. of "Corial Bottom"-containing 50 grams of resinand 400 grams of titanium dioxide. The electrode to be coated is made the anode in the coating cell and by applying to the cell a coating voltage of 30 volts for one second, a layer approximately .00025" thick is deposited on the electrode. A layer .001" is obtained when a coating voltage of 60 volts applied for two seconds is used.

A layer deposited from the above coating solution contains approximately 80% (by weight) titanium dioxide and 20% binder. Such a layer has a dielectric constant of approximately 20, a power factor less than 0.5%, a resistivity greater than 100,000 megohms and a voltage breakdown strength in excess of 1000 volts per mil thickness.

Other frits can also be used for obtaining good dielectric layers, for example, a layer consisting of 85% talc as a frit and 15% "Corial Bottom" as the elastomer binder, has a resistivity in excess of 150,000 megohms and a power factor at radio frequencies of less than 0.25%.

S60 While I have described my invention by means of specific examples and in a specific application, I do not wish to be limited thereto as various modifications will occur to those skilled in the art without departing from the spirit and scope of S65 the invention.

S What I claim is: 1. The method of applying to a flexible electrical conductor an insulating layer comprising the steps, forming in a susepnsion medium a susL 70 pension of a finely divided, refractory dielectric S material and a finely divided acrylic ester resin, S said refractory material and resin being in said suspension in a ratio greater than 5 to 1 respectively and less than about 20 to 1 respectively, s 57 and electrophoretically depositing from said suspension a layer on said flexible electrical conductor which comprises in excess of 80% by weight of said refractory material.

2. The method of applying to a flexible electrical conductor an insulating layer comprising the steps, forming in a suspension medium a suspension of a finely divided refractory material, comprising a vitreous enamel, and a finely divided elastomer, said refractory material and elastomer being in said suspension in a ratio greater than to 1 respectively and less than 20 to 1 respectively, and electrophoretically depositing from said suspension a layer on said flexible electrical conductor which comprises in excess of 80% by weight of said refractory material.

S3. The method which comprises fusing the insulating layer of a coil of the insulated electrical conductor referred to in claim 1.

4. The method which comprises fusing the ing sulating layer of a coil of the insulated electrical conductor referred to in claim 2.

5. A highly flexible, insulated, electrical conductor produced in accordance with the process defined in claim 1.

6. A highly flexible, insulated, electrical conductor produced in accordance with the process defined in claim 2.

PRESTON ROBINSON.